1846.] 



SALTER ON FOSSIL CHITONS. 



49 



accompanied by differences in the structure of tlie mantle and the 

 thickness or expansion of the entire form. 



Lamarck had long ago described a tertiary species, C. grignonensiSy. 

 figured by Deshayes ; and Cantraine, in his * Malacologie Mediterr. 

 et littorale,' added another, C. subapenninus, from Italy. As both 

 these belong to the common form of Chiton, no more need be said of 

 them. Count Miinster in his ' Beitrage' first described a species 

 from the carboniferous strata of Tournay, under the name of Chiton 

 priscus, and from detached valves, which are very numerous there, 

 he reconstructed the shell (fig. 2). Dr. Sandberger added two species 

 from the Devonian rocks of Vilmar, C.fasciatus and C. subgranosus, 

 and subsequently De Koninck figured two more new ones, C. gem- 

 matus and C. concentricus^ adding a third, C cordifer^ which however 

 by his own consent is now admitted to be an Encrinital plate. These 

 eight species were all that were known fossil, until very lately the 

 Baron de Ryckholt described ten additional ones from the carboni- 

 ferous and Devonian rocks of Tournay and Vise. The notice of 

 these appears in an elaborate paper on the external structure of the 

 shell, in which rules are given for reconstructing the entire shell, 

 when only one or part of one cerame or plate is discovered*. 



Fig.l 



Fig. 2 



Fig. 4. 



Fig. 1. Chiton Cumingii. 



2. Helminthochiton priscus. 



3. H. eburonicus. 



Fig. 4. Chitonellus. 



5. Chiton amiculatus. 



6- HelmJnthochiton Griffithii. 



With two exceptions, all the Devonian and carboniferous species 

 (figs. 2, 3) much resemble each other, and their relations to living 

 forms seem only to be with such as C. incisus and C. alatus of the Phi- 

 lippines, in which the plates are lengthened, and their contour square 

 instead of transversely oblong. The ordinary form of Chiton is seen 

 in fig. 1, C. Cumingii, a species from Valparaiso, and may be re- 

 cognised in the common C. cinereus of our own shores ; the plates 

 are transverse, and the sides are marked by a diagonal line, fig. 1 a, 

 the ornaments behind which are of a different character to those in 

 front of it. The two areas {d and e) are called respectively the 

 dorsal and lateral areas. The under surface is marked for the attach- 

 ment of muscles, and is imbedded in the surface chiefly by an ex- 



* Bulletin de I'Acad. Royale de Bruxelles, torn. xii. 2nde partie, 1845, p. 36. 

 VOL. III. PART I. E 



