But the antorbital vacuities between the nasal, pre-frontal, and maxillary bones are the sole external nostrils in the Plesiosaurs ; the zygomatic arch abuts against the fore part of the tympanic, and fixes it. A much greater extent of the roof of the mouth is ossified than in lizards, and the palato maxillary and pterygo-sphenoid fissures are reduced to small size. The teeth, finally, are implanted in distinct sockets. That the Plesiosaur had the "head of a lizard" is an emphatic mode of expressing the amount of resemblance in their cranial conformation. The crocodilian affinities, however, are not confined to the teeth, but extend to the structure of the skull itself.
In the simple mode of articulation of the ribs, the lacertiau affinity is again strongly manifested ; but to this vertebral character such affinity is limited ; all the others exemplify the ordinal distinction of the Plesiosaurs from known existing reptiles. The shape of the joints of the centrums ; the num ber of vertebrfe between the head and tail, especially of those of the neck ; the slight indication of the sacral vertebrae ; the non-confluence of the caudal hiemapophyses with each other, are all " plesiosauroid." In the size and number of abdominal ribs and sternum may perhaps be discerned a first step in that series of development of the htemapophyses of the trunk which reaches its maximum in the plastron of the Chelonia.
The connation of the clavicle with the scapula is common to the Chelonia with the Plesiosauri ; the expansion of the comcoids —ex treme in Plesiosauri —is greater in Chelonia than in Crocodilia, but is still greater in some Lacertia. The form and proportions of the pubis and ischium, as compared with the ilium, in the pelvic arch of the Plesiosauri, find the nearest approach in the pelvis of marine Chelonia; and no other existing reptile now offers so near, although it be so remote, a resemblance to the structure of the paddles of the Plesiosaur. Amongst the many figurative illustrations of the nature of the Plesiosaur in which popular writers have indulged, that which compares it to a snake threaded through the trunk of a turtle is the most striking ; but the number of vertebra: in the Plesiosaur is no true indication of affinity with the ophidian order of reptiles.
The reptilian skull from formations underlying the Has, to which that of Plesiosaurus has the nearest resemblance, is the skull of the Pistosaurus ; in this genus the nostrils have a similar position and diminutive size, but are somewhat more in advance of the orbits, and the premaxillaries enter into the formation of their boundary : the premaxillary muzzle and the temporal fosste are also somewhat longer and narrower. The post-frontals and mastoids more clearly combine with malars and squamosals in forming the zygomatic arch, which is of greater depth in Pistosaurus ; the parietal foramen is larger ; there is no trace of a median parietal crest. On the palate, besides the vacuities between the pterygoids and pre sphenoids, and the small foramina between the palatines, pre maxillaries, and maxillaries, there is in Pistosaurta a single median foramen in advance of the latter foramina, between the pointed anterior ends of the pterygoids and the premaxil laries. In Nothosaurus the pterygoids extend back, under
lapping the basi-sphenoid, as far as the basi-occipital, the median suture uniting them being well marked to their ter inination ; and there is no appearance of vacuities like the pterygo-sphenoid ones in Plesio- and Pisto-scturus. The tym panics are relatively longer, and extend farther back in Pisto than in Plesiosaurus. There is no trace of lacrymals in Pistosaurus; and its maxillaries are relatively larger than in Plesiosaurus. In Pistosaurus there are 18 teeth on each the upper jaw, including the 5 premaxillary teeth ; in Plesiosaurus there are from 30 to 40 teeth on each side. In Pistosaurus the teeth are relatively larger, and present a more oval trans verse section: the anterior teeth are proportionally larger than the posterior ones than they are in Plesiosaurus. The dis proportion is still greater in Nothosaurus, in some species of which the teeth behind the premaxillary and symphysial terminal expansions of the jaws suddenly become—e.g., in Nothosaurus mirabilis (fig. 68)—very small, and form a straight, numerous, and close-set single series along the maxillary and corresponding part of the mandibular bone.
Both Nothosaurus and Pistosaurus had many neck-verte brae, and the transition from these to the dorsal series was effected, as in Plesiosaurus, by the ascent of the rib-surface from the centrum to the neurapophysis ; but the surface, when divided between the two elements, projected further outwards than in most Plesiosauri.
In both Notho- and Pisto-saurus the pelvic vertebra develops a combined process (par- and di-apophysis), but of relatively larger, vertically longer size, standing well out, and from near the fore part of the side of the vertebra. This process, with the coalesced riblet, indicates a stronger ilium, and a firmer base of attachment of the hind limb to the trunk, than in Plesiosaurus. Both this structure, and the greater length of the hones of the fore arm and leg show that the muschelkalk predecessors of the liassic Pl&siosauri were better organized for occasional progression on dry land.
More than twenty species of Plesiosaurus have been de scribed by, or are known to, the writer ; their remains occur in the oolitic, Wealden, and cretaceous formations, ranging from the lias upwards to the chalk, inclusive. A comparison of remains of various Ple.siosauri has led to a conviction, that specific distinctions are accompanied with well-marked dif ferences in the structure and proportions of answerable verte brae, but are not shown in small differences of number in the cervical, dorsal, or caudal vertebra.
When any region of the vertebral column presents an unusual excess of development in a genus, such region is more liable to variation, within certain limits, than in genera where its proportions are more normal. The differences of the number of cervical and dorsal vertebra, ranging between 29 and 31 in the Plesiosaurus Hawkinsii, e. g.—as noted in the description of that species in the writer's Report on British Fossil Reptiles, 1839—indicate the range of variety observed in the only species of which, at that time, the vertebral column of different individuals could be compared.