32 Proceedings of the Royal Physical Society. 



the next ; on the dorsal aspect close behind the carapace is 

 also a small fin (PL I., Fjg. 3). i\long the dorsal margin the 

 scales are different in shape from those on the sides ; in front 

 of the fin they seem to be in the form of a few narrow, longi- 

 tudinal, median plates; behind it they are elongated and 

 imbricating, the arrangement reminding us of the so-called 

 fulcra or V-scales along the extremity of the tail of an 

 Acipenseroid fish ; but whetlier they are monostichous or 

 distichous it is hard to determine. The external sculpture of 

 the scales is rarely seen, and can therefore hardly be available 

 as a specific character. (See Agassiz's figure of the scales 

 of Pt. eormUus in 4, pi. ii., fig. 3.) 



The Jin is triangular-acuminate in shape, and seems to 

 have been covered with small scales, no distinct rays being 

 seen. At least two specially prominent elongated scales are 

 placed along its anterior margin, producing an appearance 

 Avhich has been mistaken for that of a spine. The position 

 of this fin is undoubtedly dorsal, as held by Hugh Miller, 

 and not anal, as supposed by M'Coy (6, p. 599). Sir Philip 

 Egerton supposed that in addition to the dorsal two ventrals 

 were also present (9 a, p. 127) ; but having examined the 

 specimen, now in the British Museum, on which he founded 

 this conclusion, I find that the two supposed ventrals are 

 merely parts of the dorsal separated by a little fault or 

 dislocation in the stone. 



As regards the British species of Pterichthys I have already 

 indicated my views in the Geological Magazine of this month. 

 Their characters, so far as I can see, are entirely dependent 

 on slight differences in the shape of the carapace and of the 

 terminal segment of the arm, so that I have often suspected 

 that after all only one " good " species was really represented. 

 Were this view to be adopted, then the name Pterichthys 

 Milleri, Ag., would include all the others as varieties. 



AsTEROLEPis, Eichwald (published April 1840). 



{ — Astcrolcjns, Agassiz, pars, non Hugh IMiller ; Fterichthys, Owen, 

 "W'hiteavcs, et cet. auct. pars, uon Agassiz.) 



AVe have seen that Pander maintained the identity of 

 Asterolejns, Eichwald, with Pterichthys, Agassiz ; and as the 



