1905.] OF ELASMOBRANCH FISHES. 45 



were entirely absent. In two much larger specimens (47 cm. and 

 58 cm. long respectively), studied by Steinhard, denticles were 

 found to be present on the mucous membrane investing the 

 gill-arches. 



Fam. Heterodontid^. 



11. Heterodottus (Cestracion) philippi (BL). — In a ma^seum 

 specimen, measuring 60 cm. in length, coarse denticles were pre- 

 sent on the roof and floor of the mouth and pharynx, but as the 

 specimen was not available for dissecting purposes I was unable 

 to determine the precise limits of their distribution. 



Fam. Spinacid.'E. 



12. Centrina salviani Risso. — In an example 23 cm. long oral 

 and pharyngeal denticles wei'e found to be entirely wanting. 



13. AcANTHiAS VULGARIS Risso. — An examination of several 

 specimens of this fish, whose length averaged about 60 cm., showed 

 that denticles were present over the floor of the mouth and pharynx, 

 and extended from the latter on to the mucous membrane covering 

 the branchial arches, where they extended as far as the bases of 

 the gill-filaments and even over the gill-rakers also. In a young 

 fish, 26 cm. long, the denticles had not yet appeared above the 

 surface of the mucous membrane, with the exception of a small 

 patch over the region of the basi- branchial cartilage. A portion 

 of the mucous membrane, from the floor of the pharynx of this 

 species, containing denticles, is represented in PI. III. fig. 4. 



Fam. Rhinid^. 



14. Ehina squatin^a L. — In an example of this species measuring 

 80 cm. long denticles were found sparsely scattered in an irregular 

 manner over the roof and floor of the oral cavity, and they 

 extended also on to the pharyngeal margins of the hyoidean and first 

 branchial arches. The denticles of this species are very remarkable 

 in their form {vide PI. III. fig. 5). Each consists of a large basal 

 plate, irregular in its outline, and in its centre is a boss-like pro- 

 tuberance which has its surface intersected by several blade-like 

 ridges. The protuberance appears to be the last remnant of the 

 spinous portion of the denticle which attains its full development 

 in the placoid scales of the skin. 



Steinhard deals with this species in considerable detail, and he 

 regards the denticles as being placoid scales which have not reached 

 their full development owing to an insufficient supply of lime salts. 



B. Batoidei. 

 Fam. RhixObatid^. 



15. Rhinobatus productus Girard. — In this species, closely 

 arranged denticles completely invest the lining of the mouth and 



