30 BULLETIN OF THE 
The name Didymodus having been proposed as a substitute for 
Diplodus Ag., it passes out of use as a synonym for Xenacanthus Beyr. 
Such being the case, it is not available as a name for a new genus. 
Professor Cope suspects that the skull he has described represents a 
different genus from Pleuracanthus proper (Xenacanthus Beyr.), which 
“will not differ from Chlamydoselachus Garm.” As it does differ very 
decidedly from the latter genus, confusion will be obviated by allow- 
ing the name Didymodus to remain a synonym, and employing a new 
name, say Diacranodus, for the new genus. ‘The species will then stand 
D. compressus and D. platypternus. The genus may be distinguished 
by the attachment of the pterygo-quadrate to the post-orbital process of 
the cranium, and by the teeth: cusps two, diverging, subconical, slender, 
and separated by a median rudimentary denticle or button on the base ; 
bases extended backward, thinner and rounded posteriorly. 
By reference to page 25, it will be seen that, while on some points we 
accord well, in others there is considerable variance between Professor 
Gill’s views and my own. Against his assertion that Cladodus is not at 
all related to Chlamydoselachus, and that it did not have the essential 
dentition of the latter, 1 am compelled to maintain that these genera 
have essentially the same dentition, that they are very closely related, 
and that, in fact, the recent genus is the living representative of the 
fossil Cladodus. As the letters and replies noted in the history, pages 
22-25, discuss the question, it is not necessary to give it further atten- 
tion here. In regard to the recent affinities our differences are slight. 
The arrangement given below expresses the conclusions reached as nearly, 
perhaps, as any at present made. A comparison with Professor Gill’s 
letter in Science, March 21, 1884, will show the extent of our agree- 
ments and differences. 
Hybodonti: Hybodus and allies. 
Cladodonti: Cladodus and allies ; 
( SELACHII 4 eee spd) Chlamydoselachus. 
Notidanit; Hexanchus and Hepta- 
EHlasmobranchii etl cee branchias. 
( I. | Heterodonti: Heterodontus (Ces- 
HOLOCEPHALL, oiesvciuese 
Lamne: Most living sharks. 
Rhine (Squatine). 
Without resorting to the “genealogical tree,” a scheme that shall not 
separate groups that belong to the same line of descent is difficult to 
arrange. In the above the Hybodonti were probably the progenitors of 
the Heterodonti and a portion of the Lamnez. The Rhine may have 
