76 THE COAL MEASURES AMPHIBIA OF NORTH AMERICA. 



The order Microsauria was established by Sir William Dawson in 1863 (208) as 

 a family of "reptiles" for the reception of the genera Hylonomus, Ilylerpeton, Smi- 

 lerpeton, and Fritschia. Hylonomus lyelli is the type species of the order. Dawson 

 (216, p. 635) says of the species Hylonomus lyelli Dawson: "It is the type of the 

 genus Hylonomus and of the family Microsauria." The forms which have been 

 referred to the genus Hylonomus, and hence to the order Microsauria, from the 

 deposits of Europe are discussed under Hylonomus. 



The Microsauria have been regarded by the writer and others as being ancestral 

 in a sense to some of the later reptiles (469) , but there seem to be insuperable obsta- 

 cles in the way of a direct derivation of the reptiles from the Microsauria. One of 

 these obstacles seems to be found in the structure of the hand. In all Microsauria, 

 so far as is known, there is no evidence of more than 4 digits in the hand, while no 

 true primitive reptile possessed less than 5. The carpus of all true reptiles is osseous, 

 while that of the Microsauria is merely cartilaginous. It is possible that the Micro- 

 sauria stand in some such ancestral relation to the later reptiles as the Crossoptery- 

 gia (4896) do to the Amphibia. The Microsauria had undergone adaptive modifica- 

 tions as to structure and habit, so that they have paralleled many of the groups of 

 reptiles, but their structure is quite different. The evidence, as far as we can see 

 now, points to a close genetic relation between the reptiles and the Microsauria, but 

 that this relation is ancestral I, for one, am not ready to say. 



The Group Aistopoda Miall, 1873, is untenable. 



The group Aistopoda was established in 1873 as section ix by the Committee of 

 the British Association for the Advancement of Science, in their "Tabular View of 

 the Classification of the Labyrinthodonts." L. C. Miall (449, 450) was the secre- 

 tary of the committee, and the report was published in two parts. Two genera 

 were at that time attributed to the Aistopoda, Ophiderpeton and Dolichosoma, both 

 described by Huxley from the Coal Measures of Kilkenny, Ireland. 



Fritsch (251, pp. 107-126) in 1883 refers to the group as "Familie" and describes 

 4 genera and 9 species as belonging to the group. Zittel (642, p. 383) refers to the 

 group as "Familie" and places 5 genera in it. Smith- Woodward (Vertebrate Pale- 

 ontology, 1898, p. 129) refers the Aistopoda to a suborder. In Eastman's trans- 

 lation of Zittel's Paleontology the group is called a family, " Aistopodidee. " Lydek- 

 ker (393, p. 205) regards the group as a suborder. The writer (469) refers the 

 Aistopoda to an order. The group Aistopoda has been adopted by practically all 

 paleontologists and zoologists who have had occasion to refer to these animals. 



Lydekker (393) in 1890 defined the group as follows: 



"Body long and snake-like, without limbs, and apparently without pectoral or pelvic 

 girdles. Vertebrae with elongated centra and aborted neural spines. Ribs slender, and 

 barbed like those of fishes. Teeth smooth, without plications of the dentine. External 

 gills probably persistent." 



If now we take up a consideration of each of the characters mentioned by Lydek- 

 ker we find that the first one holds good for all examples of the group. The second 

 character, "without limbs," is not good. Species of (Estocephalus, Ptyonius, Molgo- 

 phis all possess limbs; and doubtless Ophiderpeton will be found to possess limbs 



