50 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuDC 17? 



pansion of the forward edge, divided into two rounded lobes (h, b) 

 and separated by a toothed central portion. These sharp edges or 

 sustentacula (termed by De Ryckholt apophyses) are important to 

 notice, and are very large and remarkable in some of the fossils before 

 us (fig. 3) and in Chitonellus (fig. 4). The extent to which the 

 mantle covers the upper surface varies in different groups of the 

 genus: in fig. 1 it extends but just over the edge, in many common 

 species much further, and so as to leave only a rhomboidal surface 

 free, the covered part being smooth, and always at a lower level than 

 the free portion. In C. incisus, which has a great general resemblance 

 to fig. 1, it does not extend very far over, but has a great expansion 

 beyond the shell ; in C. amicidatus, fig. 5, the shell is wholly con- 

 cealed by it ; in C.porosus and C. monticnlaris a narrow ridge alone is 

 free, and the mantle is so thin, that the rugosities of the shell are seen 

 beneath it. 



There is one more peculiarity in the formation of the shell that is 

 worth notice, viz. the inflected portion beneath the apex of each plate 

 (fig. 2 i i), which diminishes in size according as the plates are more 

 closely pressed against each other. Now the difference between the 

 cephalic plate (fig. 1 c) and those on the back, consists in the former 

 wanting the distinction of lateral and dorsal areas, for this reason, 

 that the dorsal area is the space covered by each preceding plate 

 from its earliest size, and the anal plate (e) differs from the dorsal 

 ones only in having the area behind the apex turned out instead of 

 inflected, being then ornamented in the same way as the lateral area 

 of the other plates, which are in fact portions of a similar surface. 



In the elongated form of Chiton, of which C. incisus may be taken 

 as a type, we have the plates as long as or longer than they are 

 wide, the apex of each plate a little produced behind, and that of the 

 anal plate (fig. 2 a) carried back considerably, not, as in fig. 1, brought 

 up close to the penultimate plate. The mantle too is widely ex- 

 panded, smooth, and covering only the anterior corners of each plate. 

 The sustentacula (b, b) are of moderate size and widely separated 

 in this group. That the smooth plates of our palaeozoic fossils were 

 not covered by the mantle, I have reasons for believing certain, and 

 therefore we have a tolerable approximation to a rare modern form. 

 But in such species as C.ffemmatus, DeKon., and C.eburonicus(fig.S) 

 we have a departure from this type so considerable, that I am inclined 

 to believe it a distinct subgenus, connecting those last mentioned with 

 Chitonellus, Lam. (fig. 4), in which the plates are inserted at a di- 

 stance from each other, their form being that of an elongated rhomb, 

 and the sustentacula occupying the larger part of the plate. The 

 fossils of this section however (fig. 3), though decidedly approaching 

 Chitonellus, the lateral area being undistinguishable from the dorsal, 

 have the sustentacula widely separated and the surface granulated 

 equally all over. In C, tornacicola and C. Scaldianus, De Ryckh. 

 (minute carboniferous species), the form is much more nearly that of 

 ordinary Chitons, and the lateral area is marked by a faint row of 

 granules. 



The Silurian fossil however (fig. 6) differs essentially from those 



