ICHTHYOPSIDA. 



393 



1876; Theriodontia, Journal Geol. Soc. xxxii. 1876; Dicynodon, Id. Trans. Geol. 

 Soc. vii. 1845-56, and Ph. Tr. 1862 ; in Elgin, Judd, P. R. S. xxxix. 1885. Deino- 

 sauria, Marsh, Nature, xxxi. 1884-85 ; Papers in American Journal of Science 

 and Art by same ; and by Cope in Proc. Philadelphia Academy, &c. For Iguanodon, 

 Dollo, Bull. Mus. Royale d'Hist. Nat. Belgique, ii. and iii. ; Moseley, Nature, xxviii. 

 1883. Pterosauria or Ornithosauria, Seeley, J. L. S. xiii. 1878; H. von Meyer, 

 Fauna der Vorwelt, 1860 ; Zittel in H. von Meyer's Palaeontographica, xxix. 1882- 

 83; Marsh, American Journal of Science, xxiii. 1882. Fossil Crocodiles, see 

 Nature, xxxiii. 1885-86, p. 331. 



Figures of many Reptilian bones will be found in Phillips' Geology of Oxford 

 and the Thames Valley, Oxford, 1871. 



ICHTHYOPSIDA s. Anamniota s. Anallantoidea. 



THE amnion is absent altogether, and the allantois, when represented, 

 _never extends beyond the body-walls as a foetal membrane. Other import- 

 ant characters are the following. 



The epidermic exoskeleton is either absent, or takes the form of 

 enamel coating a subjacent dermic exoskeleton of bone. The cells of 

 the epidermis are connected by protoplasmic processes to one another. 

 The cutis is composed of vertical and horizontal connective tissue bundles 

 as in Reptilia. There is a series of epidermic sense-bodies or nerve- 

 eminences forming ' the system of the lateral line ' which may be aborted 

 in the adult. The integument forms a median fold extending along the 

 back, round the tail, and as far forwards as the anus, very rarely in front 

 of it. This fold is the azygos fin. It may persist as a continuous fold ; 

 be broken up into sections, the dorsal, caudal and anal fins ; or else be 

 aborted. The chondrocranium is often large. The basisphenoid bone is 

 either absent or inconspicuous. Of the investing bones the parasphenoid, 

 when present, is large. There are at least four pairs of branchial arches 

 present in development, and the number may be greater. They are either 

 retained throughout life, or considerable remnants are generally to be 

 found in the adult. The notochord may or may not persist wholly or in 

 part. There is no costal sternum, i. e. one formed from the ventral ends of 

 the primitive ribs. The spinal accessory nerve is a branch of the vagus : 

 the hypoglossal represented by the first, or first and second spinal nerves. 

 The digestive tract either ends in a cloaca, or the anus opens in front of 

 the urino-genital apertures. The heart consists of a sinus venosus, one or 

 two auricles, a ventricle and a conus arteriosus the last-named being 

 aborted in Teleostean Fish. A pair of aortic arches at least persists : and of 

 the others present in development all or a certain number are temporarily 

 or permanently connected with gills. The haematids are large, oval and 

 nucleated. Branchiae external or internal are present either throughout 

 life or during its first stages. The ova are generally numerous. 



There are three classes, Amphibia, Pisces and Cyclostomi. 



