Transversely segmented animals with a hard shell of external covering (chitin) and the various subdivisions include the (A) Crustacea, such as crayfish, which are aquatic, carnivorous animals. The Trilobites are a subclass of the Crustacea and other similar sub-divisions of it are the Phyllopoda which are small, elongate, fresh-water animals. The Ostracoda are minute brackish, fresh-water, as well as marine creeping animals. The Copepoda are unknown as fossils. Cirripedia or Barnacles are degenerate form of Arthropoda, either fixed or parasitic marine forms. Malacostraca consist of thorax divided into eight, and the abdomen into seven segments. It will be noticeable that the Trilobites are strictly Paleozoic forms, becoming extinct at the end of the Permian.
Subdivision (B) Onychophora have no fossil representatives. (C) Myriopoda have a worm-like body with many similar segments bearing legs (Centipedes). (D) Arachnida, of which the Spider is a typical example, have several orders of which Eurypterida is the most important. (E) Insecta. Insects, bugs, flies, etc., are classified under this heading.
Life has reached the highest form in this phylum, the maiu advance being in the development of a second body-cavity in which the nervous system is arranged, known as the spinal cord.
The members of this phylum are known as Vertebrates to dis tinguish from the members of other phyla, which are referred to as Invertebrates.
The first sub-phylum Adelochorda have no fossil representa tives. Urochorda are a degenerate form of Chordata, also unknown in the fossil state. The third sub-phylum is the main group and are divided into several sub-divisions, the two main subdivisions being first, the Acrania, or animals whose true head is absent, brains but slightly developed and no heart present. No fossil remains of this division are known. The second or Craniata is composed of animals having a true head (skull) highly developed brains, pair of complex eyes and heart and blood with red corpuscles.
The Craniata are the most important of the Vertebrates and its subdivisions are as follows: (A) Cyclostomata is composed of degenerate eel-like fishes.
(B) Ostracodermi, are animals having a large shield or armor composed of several pieces completely enclosing the head.
(C) Pisces or fishes.
(D) Amphibia. They differ from fishes in having paired five-toed limbs instead of fins; scaly or bony covering. The order Stegocephalia are extinct tailed amphibia often of great size, living mostly in fresh water and some may have been terrestial. Urodela fossil remains are rare; they include the lizards. Anura are the frogs and toads. The Gymnophiona are snake-like without limbs; fossils are rare.
(E) Reptilia are cold-blooded vertebrates with two pairs of limbs with five toes; also having a horny or scaly skeleton. Some large specimens of this class are extinct and among the aquatic forms Ichthyosaurs are well-known examples. Of the land reptiles Dinosaurs (Triassic to Cretaceous) were a familiar type. The Pterosauria (Flying reptiles) appeared in the Jurassic and became extinct in the Cretaceous, Ptcrodactylus well-known fossil representative of this group. The crocodila reptiles are
at present represented by the crocodiles and alligators, there are, however, several other extinct members of this branch. The order Chelonia includes reptiles having a horny or hard bony covering completely enveloping the body as in turtles. The order Squamata includes reptiles with an external protection of horny scales, and snakes are the most common example of this order.
(F) Ayes (Birds). Although Mesozoic birds had functional teeth, since the Tertiary no teeth are present. The oldest known bird is Archeopteryx of the upper Jurassic. The remains of birds are rarely found as fossils and of Archeoptcryx only very few good specimens have been found.
(G) Mammalia (Mammals). These are air-breathing, warm blooded vertebrates usually with an external covering of hair. For general discussion they are divided as follows: (a) . Monotremata, of which only two remaining members are known, which arc Ornithorhynchus (Duck Bill) and Echidna (Spiny Anteater). The young arc produced as eggs and are carried in a ventral pouch, where they arc hatched and nourished. (b) . Marsupialia produce their young alive, but are carried in a pouch until they arc able to take care of themselves; exam ples are the opossum and kangaroos.
(c). Insectivora are insect or worm caters.
(d). Chiroptera have the fore limbs modified to form wings. (e) . Carnivora are flesh eating mammals with teeth having sharp cutting edges.
(f). Rodentia are fur covered, plant-eating animals.
(g) . Edentata are degenerate forms with imperfect teeth.
(h). Ungulata or hoofed mammals, with none or small canine teeth,with large premolars and molars, flat footed animals, such as elephants and similar extinct mammals, as well as cattle,sheep, etc.
(i). Sirena or sea cows (dugong).
U). Cetacea are the whales.
(k). Primates, are walking mammals, with thumb and digits having flat nails, eye surrounded with bony ring. Lemuroidea are the lemurs, which are the only living survivals or a group of extinct similar mammals. Anthropoidea, which have several distinct classifications amongst which are the apes, monkeys as well as similar extinct forms. The final classification .being the various extinct as well as living family of Hominid, or man, with modern man (Homo sapiens) at the top of the scale.
The length at which this subject is treated here does not permit a thorough understanding of paleontology and is simply a short resume of the various facts as treated in volumes pre pared for the study of paleontology. The facts given are in part from Shimer's " Introduction to the Study of Fossils," in which these principles are taken up in greater detail. The list of fossils is by no means a complete one, but merely a list of some of the best-known representatives. The geologist should be equipped with a reference volume by means of which he may identify any fossils unknown to him. For this purpose Grabau & Shimer's "Index Fossils" will be of great value.