MUSEUM OF COMPARATIVE ZOOLOGY. 67 



A connection of the cranials, instead of the laterals, by the aural, 

 and the passage of the jugulars across the chest, at once separate the 

 Holocephala from both the Galei and the Batoidei. The Batoidei are 

 separated from the Galei by the possession of the pleural canals. 



Chimsera, in the Holocephala, is marked by the grooves, instead of 

 tubes, and Callorhynchus by the tubes, instead of grooves. 



Among the Galei, on the base of the tail the lateral canals descend 

 to the lower edge of the muscles in Chlamydoselachus, Heptabranchias, 

 Heterodontus, Pristiophorus, Acanthias, and Somniosus, as in the Holo- 

 cephala. Open corporal canals resembling those of Chimsera appear on 

 Chlamydoselachus, Heptabranchias maculatus, and, in part, on Acanthias. 

 On other genera the laterals maintain their position near the vertebrae 

 of the tail, and the canals are tubular. On Scoliodon, Mustelus, and 

 the Hammerheads the lateral makes a decided bend below the second 

 dorsal fin ; and it ends at or near the end of the vertebral column in 

 Scoliodon, Triacis, Mustelus, Odontaspis, Scylliorhinus, Chlamydose- 

 lachus, Ginglymostoma, Cestracion, and Somniosus, not reaching so 

 far back in others. Disregarding the course of the column in Isurus, it 

 passes directly backward, ending at the edge of the muscles just above 

 the lower lobe of the caudal fin. 



The aural is behind the "ear openings," and more or less curved back 

 in the majority of the Sharks ; it is in front of the openings in Chlamy- 

 doselachus, bisected in Heptabranchias, curved forward in the middle 

 in Acanthias and Chlamydoselachus, and nearly straight in Mustelus, 

 Scylliorhinus, Heterodontus, and Somniosus. Sometimes, as in Pristio- 

 phorus, it is deeply curved backward, much as in the Holocephala, or 

 in Dicerobatus. 



The occipitals appear like continuations of the laterals, so slight is 

 their change in direction, in Acanthias, Rhina, Heptabranchias, and 

 Chlamydoselachus ; others have the tubes directed more or less obliquely 

 outward. 



Somniosus is peculiar in that cranials, orbitals, and occipitals do not 

 meet on the crown. 



On the frontal region the cranial curves are shallow in Prionodon, 

 Alopias, Isurus, Heterodontus, Acanthias, Somniosus, and Pristiophorus; 

 decided in Scylliorhinus, Mustelus, Triacis, Ginglymostoma, and Rhina ; 

 more decided in Chlamydoselachus and Scoliodon ; and excessive in 

 Cestracion (Zygasna). 



A majority have the orbital bent forward in its lower portion ; in 

 Cestracion, Heptabranchias, and Chlamydoselachus it bends backward. 



