FAMILY M.-RAM A.
(Rays.) Fossil evidences of this peculiar family of cartilaginous fishes have been discovered in oolitic (Spathobatis Belem nobatis), cretaceous, and tertiary formations, and consist of defensive spines, dermal tubercles, and teeth, but chiefly the latter. The most peculiar and distinctive modifications of the dental system, presented by the eagle-rays (Hyliobaticice) are unequivocally shown by fossils of the tertiary formations, and have not been found in earlier strata.
The teeth of the rays are in general more numerous than those of the sharks ; they have less mobility, are more closely impacted, and in some cases are laterally united together by fine sutures, so as to form a kind of mosaic pavement on both the upper and lower jaws. The Myliobate8, or eagle-rays, which present the last-mentioned condition, unique in the ver tebrate sub-kingdom, have large and massive teeth (fig. 38) ; but in the rest of the present family of cartilaginous fishes, the teeth (fig. 38) are remarkable for their small size as compared with those of the sharks. The teeth in some species of rays are adapted for crushing, but in others they have the middle or one of the angles of the crown produced into a sharp point. In all genera of the ray tribe, whatever the diversity of size and shape of the teeth, they are placed in several rows, and succeed each other uninterruptedly from behind.
The modification of the plagiostomous type of teeth, for the purpose of crushing alimentary substances, is most complete in the genus Myliobates. A view of this armature of the mouth, as seen from behind in the Myliobates aquila, is given in fig. 38. Both jaws are covered with a pavement of broad teeth, Jaws and teeth of an Eagle-Ray (Myliobates aquila).
having a flat grinding surface. To the genus Myliobates, as now restricted, certain fossils from the London clay of Sheppy (Myliobates toliapicus, Ag, fig. 39) belong.
In Zygobates (fig. 40), the middle series of teeth is less broad ; and a narrower series is interposed between the mid dle and the small lateral teeth. Existing rays showing this modification are found in Brazilian seas ; fossil teeth of this genus, e. g., Zygobates Woodwardi, Ag. (fig. 40), occur in the tertiary crag (probably miocene) of Norfolk, and in the mio cene mollasse of Switzerland.
When the teeth form broad transverse undivided plates, as in fig. 41, they characterize the genus 4Etobates. Fossils of this genus occur in the English eocenes and the Swis smol lasse.
In the "crag" of Norfolk and Suffolk, and in marine plio cene beds, fossils have been found which closely resemble the osseous and spinigerous plates that beset the skin of the kind of ray called " thornback" (fig. 41 a), and which indicate the existence of a pliocene species allied to the Rata elavata.
Thus we obtain evidence of fishes of the plagiostomous order in the marine deposits of every formation from the upper Silurian beds to the present period. But none of the palteozoic fossils are referable to any existing genus. A few only of the mezozoic Plagiostomes, and those chiefly from the chalk, are so determinable : but most of them belong or are allied to a family (Cestrctciontidce), now nearly extinct. The evidence of the generic forms of Plagiostomes characteristic of the present time become common only in the tertiary periods. No fossil species is the same with any existing one.