Mat 22, 1908] 



SCIENCE 



817 



The Permian is the oldest fauna of land verte- 

 brates of which we have any extensive 

 knowledge, illustrates the early stages in the 

 adaptation of the vertebrate phylum to ter- 

 restrial locomotion and shows us what the 

 early land vertebrates were like from which 

 the various orders of reptiles and mammals 

 are more or less directly descended. It is 

 from the study of the reptiles and amphibia 

 of this period that we shall obtain the best 

 evidence regarding the relationship of the 

 several orders of reptiles and the origin of 

 the mammals. 



The typical Pelycosauria are better de- 

 scribed as archaic rather than primitive, since, 

 while for the most part remarkably primitive 

 reptiles, they are in certain respects highly 

 specialized. This is especially seen in the 

 enormously elongated neural spines of the 

 vertebrse, which form a high rigid bony fin on 

 the back, and in the differentiation of the 

 dentition by enlargement of certain teeth to 

 serve as canine teeth, as in carnivorous 

 mammals. This specialization of the denti- 

 tion is correlated with great changes in the 

 form of the skull and the proportions of its 

 bones, and the development of the back fin, 

 with reduction of the tail and adaptation of 

 th$ feet and limbs to more truly terrestrial 

 locomotion. 



The two best-known genera are Dimeirodon 

 and Naosaurus. In the first the spines are 

 very long, slender and simple; in the other 

 they are not quite so long, but stouter, and 

 provided with a series of short cross-bars like 

 the yards of a full-rigged ship. Dimeirodon 

 is knovpn from the complete skeleton; in 

 Naosaurus the proper correlation of the skull 

 is in doubt. These animals were of fairly 

 large size, six to eight feet in length, mas- 

 sively proportioned, the head high, compressed, 

 nearly a foot long in the largest species of 

 Dimeirodon, vrith fin spines over three feet 

 long. Dr. Case considers that the spines 

 must have been connected by a web of tough 

 horny skin, but were probably not covered by 

 flesh. No satisfactory explanation of the use 

 of this fin has been given ; the author regards 

 it as illustrating Beecher's law of the exuber- 

 ance of spines and bony outgrowths in the 



last stages of evolution of a race. These 

 genera are the extreme stages in the special- 

 ization of the pelycosaurs. They are con- 

 nected by a series of intermediate forms with 

 small aquatic unspecialized types related to 

 Proiorosauru-Sj Palwohaiieria and other primi- 

 tive reptiles of the Permian of Europe. 



The inclusion of the early stages o°f 

 the Pelycosaur phylum in the order Pely- 

 cosauria is, in the reviewer's opinion, open to 

 some objection. These small primitive genera 

 are structurally ancestral to the typical Pely- 

 cosaurs (they are not genetically so, as Dr. 

 Case is careful to point out, since so far as 

 we know they are all contemporaneous), but 

 that is no reason for including them in the 

 order. An order, genus or species is defined 

 by certain facts of common structural jpecul- 

 iarities indicative of descent from a common 

 ancestor and by certain acquired specializa- 

 tions indicative of similar adaptation. The 

 ancestor is a member of the phylum, but not 

 necessarily of the order; it may belong to a 

 more primitive order which has given rise to 

 one or more specialized orders of a later epoch. 

 Unless we hold to this view of their limits it 

 becomes impossible to properly define natural 

 groups. In the opinion of the reviewer the 

 Pelycosauria could be more satisfactorily de- 

 fined if these primitive genera were left out, 

 to be placed perhaps in the Protorosauria, 

 from which the Pelycosauria would be, struc- 

 turally speaking, derivable. 



The author confirms the views previously 

 expressed by himself and by Osborn as to the 

 general relationship of the order. They 

 belong to the primitive Diapsidan or Rhyn- 

 chocephaloid group of the Reptilia (super- 

 order Diaptosauria Osborn), Sphenodon being 

 the nearest modem relative. Their relation- 

 ship to the Cotylosauria is more remote than 

 supposed by Cope, and the inclusion of both 

 groups in an order Theromora is inadmissible. 

 Substantially the same views are expressed in 

 recent contributions to the classification of 

 the reptilia by Broom' and by Boulenger' as 



^ " Classifieation of the Theriodonts and their 

 Allies," Rep. S. Afr. Assoc. Adv. Sci., 1903. 



^ " On the Characters and Affinities of the 

 Triassic Eeptile Telerpeton elginense," Proo. Zool. 

 Soc. London, 1904, p. 470. 



