Cope.] 5b0 [March 7, 



the Dipnoi merely by suture or contact, with other cartilage bones. Its 

 character is therefore more nearly that of the Holocephali than of the 

 Elasmobranchi or the Hyopomata. 



In the light of the above considerations, to which sub-class must be re- 

 ferred the genus Didymodus ? Does it possess a freely articulating hyo- 

 mandibular bone, and maxillary, palatoquadrate and mandibular arches ? 

 The question must be primarily determined by these considerations, since 

 the fins and their supports are unknown to us. 



The lateral posterior processes of the skull are in its superior plane, 

 and their extremities do not present an articular facet for the lower jaw. 

 It is improbable that they were continued downwards as cartilage for the 

 former articulation, as in the Holocephali and Dipnoi. Both from the 

 presence of an articular condyle, and from the mechanical necessities of 

 the case, I have little doubt but that there was a freely articulating hyo- 

 mandibular bone. I have already described this element in fact as visible 

 in a single specimen. The choice is thus limited to the Elasmobranchi 

 and Hyopomata. It is decided in favor of the former by the absence of 

 maxillary arch and of opercular apparatus. So then Didymodus is a 

 shark, in spite of its peculiarities. Kner* speaks of the presence in the 

 nearly allied Pleuracanthus (= Diplodus), of premaxillary and maxillary 

 bones ; but this is no doubt a misinterpretation of the homologies, as he says 

 they articulate with the lower jaw. In my jaws there is but one bone on 

 each side, a palatopterygoid. 



In his researches on the structure of the skulls of sharks, Gegenbaurf 

 shows the different methods of articulation of the palatopterygoid arch in 

 the sub-class Elasmobranchi. In Heterodontus the palatopterygoid arch is 

 attached to the skull throughout by its superior border, anterior to the 

 orbit, but is free posterior to the orbit. In Hexanchus and Heptanchus 

 it is free anteriorly, but articulates by its elevated posterior portion with 

 the postorbital process. In the remainder of known recent Elasmobranchs 

 it is free throughout, and merely in contact in front. These relations are 

 also described by Huxley. % Professor Gill utilizes them as definitions of 

 three (of four) primary divisions of the sub-class Elasmobranchi, § which 

 he names the Opistharthri, (fam. Hexanchidse) ; Proarthri (Heterodon- 

 tidse) ; Anarthi (sharks proper) ; and Rhinse (Squatinas). According to 

 these definitions, Didymodus must be referred to the Opistharthri. The 

 skull, however, presents other characters which must claim attention. Its 



* Sitzungsberichte Wiener Akademie, LV, p. 540. 



fUntersuchungen zur Anatomie der Wirbelthiere, Leipzic, 1872. 



J On the Anatomy of Ceratodus. Proceedings Zool. Society of London, 1876, 

 p. 43-4, with figures. 



\ Bulletin of the U. S. National Museum, No. 16, 1883, p. 967. Gills fourth group, 

 Rhinas, does not appear to me to possess the value of the other three, nor are 

 the "Raise" and "Pristes " more distinct. I therefore propose that the order 

 Selachii, as defined in* the following pages (of the sub-class Elasmobranchi), 

 be divided into three sub-orders: Opistharthri, Proarthri and Anarthri, the lat- 

 ter to include the true sharks, the Squatinse, the sawfishes and the rays. 



