342 Kirkhy and Young — On Fossil Chitons. 



The plates discovered appear to be intermediate ones. The one that is perfect is nine- 

 thirty-seconds of an inch long and four-thirty-seconds of an inch hroad. It is orately 

 oblong in outline, ■with an anterior sinus, and is sharply ridged along the median line 

 from the apex forwards. The apex is placed about one-fifth from the anterior margin. 

 The exposed portion of the plate, or that which was uncovered by the mantle, is 

 slightly raised, anteriorly, ovate in shape, and with a pustulous surface ; the inserted 

 portion of the plate is marked with coarse, radiating strise, except in front. 



The other two plates are both imperfect in the anterior inserted portion ; but so far 

 as they are preserved, they both agree precisely with the one described. They were 

 found in the marine shales of Craigenglen. Another perfect plate appears to have 

 been found by the late Mr. Alex. Cowan of Campsie, but has been lost. 



All the plates have been found in the marine shales of Craigenglen, Campsie. 



4. Chitonellus suhquadratus, sp. nov. Plate XVI. Fig. 5. 



We designate by the above name another plate that has also occurred in the 

 Craigenglen shales, and which cannot be referred to C. Toungianus. It may be 

 described as subquadrate in outline, with both anterior and posterior margins 

 forked, but with the former more so than the latter, and with the anterior portion 

 of the plates rather narrower than the other. It is ridged or angular along the median 

 line from the apex forwards; and from near the apex posteriorly two declining angular 

 ridges diverge and run into two sharp projections that are formed by the junction of 

 the latero-posterior margins. The exposed portion of the plate is very small com- 

 pared with the portion that was buried in the animal, being not more than a fifth ot 

 sixth part; it is heart-shaped, with the pointed extremity in front; it is slightly 

 elevated above the rest of the plate, and granulated about the margin. Numerous 

 delicate striae fringe this heart-shaped area on the surface of the inserted portion of 

 the plate. Length, quarter of an inch ; breadth, one-eighth of an inch. 



Besides the species which, we have already enumerated from Scot- 

 land, Mr. John Hunter, of Braidwood, near Carluke, has obtained 

 from the main limestone of that place one or two small plates of a 

 species of Chiton, whicli we have been unable to determine correctly, 

 owing to their imperfect preservation. 



Mr. James Thomson, of Crlasgow, has also a Chiton plate in his col- 

 lection, from Carboniferous shale on the banks of the Anan, near 

 Strathanan. His specimen, so far as we remember, agrees very 

 closely in form and ornamentation with the Chiton gemmatus of De 

 Eoninck. 



It is not unlikely, when the affinities of the palseozoic Chitomdm 

 can be carefully investigated, that the generic nomenclature adopted 

 at present will require revision and alteration. Some of the species 

 described would almost appear to possess points of more than specific 

 difference from all the known recent types of Chiton, so that it may 

 ultimately be found requisite to establish new genera for their re- 

 ception. It is well, however, not to be too hasty in the formation of 

 new types ; for our knowledge of the palseozoic species is nearly in 

 every case imperfect, and in some instances the conclusions drawn 

 from the facts known may be erroneous. 



We give figures of the eight plates of the recent Chitonellus striatus, 

 Lam., to illustrate the nearer approach of the fossils we describe as 

 CMtonelU to that genus than to Chiton. From these it will be seen 

 that in Chitonellus the plates are longer than broad, while in Chiton 

 just the contrary holds ; also that the plates are not formed to im- 

 bricate on each other as in Chiton, but to be fixed isolatedly, though 

 firmly, in the mantle ; and, lastly, that much more of the plate is 

 inserted in or covered by the mantle of the mollusc. In these 



