1846.] SALTER ON FOSSIL CHITONS. 51 



above mentioned in having the plates deeply emarginate behind, and 

 as it were bent backwards, a character very rare in living species, 

 and never occurring to this extent. The shell described, of which onlj' 

 the four front plates are preserved, was not so much elongated as in 

 the group containing C. incisus (fig. 2), the plates being wider than 

 long, carinated, but not with a separate ridge along the back, and evi- 

 dently of a very thin texture, as may be seen by the broken edges on 

 the cast. In the former respect it resembles C. amiculatus (fig. 5), 

 but there is an essential difference between the smooth external shell 

 (as I believe it to be) of the fossil and the internal one of that spe- 

 cies ; for, as if to show in what light we are to regard the inflected 

 portion, in C. amiculatus it is turned outwards in all the plate 

 (fig. 5, i i), just as in other species occurs in the anal plate, and the 

 apex therefore (5 a) is within the margin. 



In our fossil the apex (6 a) is on the posterior edge, and there is 

 no expansion behind it, not even the broad double lobe that occurs 

 in C.porosus before mentioned. 



Notwithstanding the thin texture and deep emargination of the 

 plates, I have no doubt this is one of the Chitonidce. It is not quite 

 anomalous, the C alatus, Sow., a member evidently of the incisus 

 group, having an approach to the emarginate form. This latter is 

 a tolerably thin shell, though the lateral areas are marked by being 

 slightly ornamented, and by a diagonal fold ; nevertheless it more 

 closely resembles our species than any other, and has a smooth thin 

 expanded mantle. 



Mr. Gray has (I believe in MSS. only) separated the species with 

 tufted thin spines or with hairs by the name of Acanthochcetes, but I 

 am not aware that the elongate group has been recognised, and I 

 venture to propose for this genus or subgenus, and rather for the 

 fossils than the recent shells, the name Helminthochiton, from its ver- 

 miform character. 



The twenty-three fossil species will then stand as follows* : — 



Genus Helminthochiton. 



Elongate ; plates as long as wide, subquadrate, thin ; apex of the 

 anal plate remote from its front edge ; sustentacula widely separated ; 

 shell but very little covered by the mantle [mantle expanded, smooth, 

 thin]. Tropical? 



1st Section, allied to C, alatus, Sow. ..y;) 



H. Griffithii, Salter (in GrifF. Sil. Fossils of Ireland, pi. 5. fig. 5). ^^^ 



• , ";v^ 



* Not ha-sdng access to Sandberger's paper, I cannot refer his species C. fas- 

 ciaius and C. subgranosus to their proper sections. 



I had overlooked Mr. W. King's interesting discovery of a Chiton in the mag- 

 nesian limestone. His description of the anal plate in the * Annals of Natural 

 History ' (afterwards republished with a figure in Charlesworth's Geological Journal) 

 would induce one to believe his shell to be a species of Helminthochiton, and the 

 figure given with it of the somewhat keeled dorsal plates, emarginate behind, 

 would probably refer it to my first section. The Irisb fossil however has no sinus 

 in the cephalic plate, and we do not possess the anal one for comparison. 



e2 



