326 THE PEOCEEDINGS OF THE LINNEAN SOCIETY 



I have already mentioned that the chief peculiarity of the den- 

 tition of H. galeatus is the longitudinal ridge on the posterior 

 teeth ; which character, if found in fossil Plagiostomous teeth, 

 would very probably have induced Agassiz to describe them as 

 belonging to a new genus. If disregarding the anterior (tri- 

 cuspidate) and the middle (multi-cuspidate) teeth, and regarding 

 only the posterior (elongated and ridged), we compare the latter 

 with the fossil Plagiostome teeth (or to speak more correctly, with 

 the Fig of the Atlas of the Poisson fossiles of L. Agassiz) we find 

 (on plate 12 of the 3rd vol. of the Atlas) several figures which have 

 very much the appearance of the lateral teeth of II. galeatus. 

 These are the difierent teeth of Psammodus linearis, Agass. I 

 must, however, add that this resemblance struck me more from 

 looking at the illustrations (figs. 9 — 13) than from reading the 

 text (Tome III., p. 107 and 108.) I believe, however, that one 

 would be only entitled to come to a decisive conclusion after 

 examining the fossils themselves and not merely the drawings of 

 them. 



4. — Dentition of Heterodontus Francisi. Girard. 

 After I had carefully examined the form of the teeth of H. 

 FhilUpi and H. galeatus, it was very interesting to me also to 

 investigate those of H. Francisi. Unfortunately, there was 

 at my service only one specimen, which, as a "Museum 

 specimen," I could examine only from without. The anterior 

 teeth were perfectly visible, and could be sketched without in- 

 terfering with the specimen. It was otherwise with the impor- 

 tant posterior teeth, to see which, in detail, a lateral incision of a 

 few inches, from the margin of the gape into the cheeks would 

 have proved very useful. I was obliged, however, to content 

 myself with looking in through the aperture of the mouth, so 

 that the sketching of the posterior teeth was rendered a matter 

 of great difficulty. I succeeded, however, in making the accom- 

 panying sketch (fig. 37). The form of the teeth in II. Francisi 

 resembles, on the whole, that of a young H. Phillipi ; the front 

 teeth were tri-cuspidate. In the posterior rows of large teeth 

 there were no teeth so large as are found in many adult 



