K 
Cope.] 580 [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 
ina 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, Gegenbaurt 
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. Hexanchide) ; Proarthri (Heterodon- 
tidec) ; Anarthi (sharks proper) ; and Rhine (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. 
+ Untersuchungen zur Anatomie der Wirbelthiere, Leipzic, 1872. 
{On the Anatomy of Ceratodus. Proceedings Zoél, Society of London, 1876, 
p. 43-4, with figures, 
@Bulletin of the U. S, National Museum, No, 16, 1883, p, 967. Gills fourth group, 
Rhine, does not appear to me to possess the value of the other three, nor are 
the ' Rais” and ‘‘Pristes’? more distinct. I therefore propose that the order 
Selachii, as defined in the following pages (of the sub-class Hlasmobranchi), 
be divided into three sub-orders: Opistharthri, Proarthri and Anarthri, the lat- 
ter to include the true sharks, the Squatinee, the sawfishes and the rays, 
