64 BULLETIN OF THE 



pleurals and subpleurals to a Galeoid ancestor resembling Chlamy- 

 doselachus as far as the possession of spiracular and gular canals is 

 concerned, if not further. And indirectly the tendency of such consid- 

 erations is to confirm the claim elsewhere advanced that that genus is in 

 great measure to be regarded as a persistent type. 



HOLOCEPHALA. 



The great differences between Chimsera and Callorhynchus in regard 

 to rostral appendages and claspers, are in reality no greater than those 

 obtaining in their canal systems. Greater divergence than occurs in 

 these genera is not to be seen in the most dissimilar forms of the Sharks. 

 On Chimsera the canals are furrows, as on the body of Chlamydoselachus, 

 and the oral meets the angular; on Callorhynchus the canals are tubes, 

 and angular, oral, and jugular meet the suborbital independently. At 

 the first glance, the differences in the distribution of the cephalic canals 

 in the two genera appear greater than they really are. On comparison, 

 the positions of laterals, aural, occipitals, cranials, and orbitals are found 

 to be similai*. In both cases the oral and the jugular cross the median 

 line as series of pores or short grooves, the suborbital extends to the 

 end of the snout, the subrosti'als unite under the rostrum to form a 

 median, then separate to meet the nasals, and the nasals are in front of 

 the nostrils, meeting across the middle without forming a median canal 

 or prenasals. It may be added, that in both types the lateral descends, 

 above the lower lobe of the caudal fin, to the lower edge of the muscles, 

 as in certain of the lower Galei. 



Affinities with the Sharks, through ancestry, are indicated by the 

 correspondence in laterals, aurals, cranials, orbitals, angulars, and orals. 

 Special points of disagreement are seen in the union of the jugulars, the 

 prenarial location of the nasals, the absence of prenasals, the presence 

 of a median in the subrostrals, and in the connections of the occipitals. 



COMPARISONS. 



Whether the canal system is a suitable basis for homology and classi- 

 fication, either alone or in connection with other parts of the anatomy, 

 and its importance as such a basis, are to be determined by consid- 

 eration of the extent of its development and the amount of its va- 

 riability in the different types included in the class. An exhaustive 

 investigation, of the subject would naturally demand a study of the 



