MUSEUM OF COMPARATIVE ZOOLOGY. 29 



dosirenidae. The structure points to the type from which the true fishes 

 (Hyopomata) diverged from the sharks. The characters are thought to define 

 an order of the subclass Elasmobrauclii, equivalent to all other known forms. 

 To these two divisions were given the names Ichthyotomi and Selachii." 



In the Proceedings of the American Philosophical Society of Phila- 

 delphia, 1844, the skull is figured, and from a study of the illustrations 

 conclusions are reached which differ somewhat from those embodied in 

 the above description. A comparison with the plate in the Proceedings, 

 or Pal. Bull., No. 38, will show whether they can be justified. 



1. As in aged specimens of some recent sharks, the cartilage of the 

 skull is highly charged with calcareous matter. 



2. " The penetration of the granular ossification " does not distinguish 

 these skulls from those of certain Galei. 



3. The skull is unsegmented ; the lines of segmentation, so called, are 

 partly accidentals, which are not alike on the two sides of the skull. 



4. What is called one of the nares on the superior face of the muzzle 

 is not a nostril ; it is behind the nasal sac, and is the opening of the 

 preorbital, or ethmoidal, canal. 



5. The fronto-parietal fontanelle, so called, is the parietal fossa, from 

 which pass the aqueducts of the vestibule, common to all Selachians. 



6. Figure 4 of the plate should be reversed in direction, the prolonged 

 anterior portion, in the figure, should be turned backward from the in- 

 terorbital region ; thus bringing what in the figure serve as orbits behind 

 the postorbital processes. 



7. The Ichthyotomi, as based on these skulls, have not been separated 

 from the Selachii. 



8. From these skulls alone the genus to which they belong cannot be 

 separated from the Opistharthri of Gill. 



9. The skulls belong to a genus that represents a line of descent 

 closely allied to that in which we find the Notidanidse, and is not "the 

 type from which the true fishes (Hyopomata) diverged from the sharks." 



10. The genus to which these skulls belong is probably neither 

 Diplodus Ag. {Didymodus by Cope), Xenacanthus Beyr., nor Pleuracan- 

 thus Ag., but a new one. 



It was a mistake to consider the genus identical with Chlamydosela- 

 chus ; it was another to make it identical with Xenacanthus Beyr. We 

 know the genus only from the skull and teeth. The dentition affords 

 a means of comparison, which places the genus in the Cladodonti with 

 Thrinacodus and Cladodus. There is nothing in the skull, as far as we 

 know it from the description, that will place it elsewhere. 



