FROM LYME REGIS. 247 



er striae; from Hyb. reticulatus they are at once distinguished 

 by the 'bluntness of the dental processes, and the divergence 

 of the stria upon their surface : the teeth of Hyb. medius 

 differ in the indistinctness of the division of their crowns into 

 lobes, and in the regularity of the compressed cones which 

 their forms exhibit. 



By far the greater number of species of Hybodus enume- 

 rated in the * Poissons Fossils ' are merely known by the Ich- 

 thyodorulites ; and Agassiz regrets that the rare occurrence 

 of the teeth and spines under circumstances which establish 

 their relation to the same individual, compels him to intro- 

 duce a double nomenclature. It fortunately happens in the 

 present instance, that in addition to the frontal spine, both 

 dorsal were also discovered. One of these is represented in 

 the plate of the natural size, (fig. 9). Its fellow differs in 

 being about two inches longer, in presenting a much slighter 

 decrease in the width of the grooved sides, and in having a 

 smaller number of denticulations. The character of these parts 

 is so well conveyed by Mr. Sowerby's engraving, that it is 

 unnecessary for me to enter into any minute description. 



In the future identification of this species, it should be 

 borne in mind that the supposed frontal spine may be lost, 

 while the jaws or other parts of the skeleton may be pre- 

 served ; and on the other hand, this singular appendage may 

 not improbably be common to several species of the genus. 

 The circumstance of this fossil appearing to be generally un- 

 known to collectors, even in a detached state, strengthens my 

 assumption that the shark to which it belonged was not fur- 

 nished with a series of these spines, and indeed perhaps in- 

 dicates the rare occurrence of the particular species which it 

 may serve to characterise. I hope the publication of this 

 notice may draw the attention of those interested in fossil 

 Zoology to the subject, but at present Mr. Lonsdale is the 

 only person of those to whom the fossil in question has been 

 shown, who had previously seen anything of the kind; and 

 he tells me that a similar but detached fossil body, also from 

 the lias, is preserved in the Bath Museum. 



Being unable to identify the present species with any one 

 that has been described, I have called it Hybodus Delabecheii, 

 M. De la Beche having, I believe, been the first geologist who 

 drew attention to the fossil remains of the genus. 



REFERENCES TO PLATE NO. 4. 



Figs. 1 & 2. Tabular masses upon the borders of which the teeth are dis- 

 posed. The posterior and right lateral borders extend farther than re- 



