NATURAL HISTORY SOCIETY OF GLASGOW. 323 



is found in a fossil state so far back as the Silurian period, thirty- 

 seven species being known. Until the discovery of the specimens 

 now exhibited, which he (Mr Young) had found in the lower 

 limestone shales at Craigenglen, Campsie, no fossil specimens of 

 the sub-genus Chitonelliis had been recorded for Scotland. The 

 recent living representatives of this sub-genus, of which ten 

 species are known, are said to be found in the fissures of coral 

 rock, and their present distribution is in the seas of the West 

 Indies, Western Africa, the Philippines, Australia, the Pacific, 

 and Panama. It is interesting, therefore, to find that they 

 have lived so far back in time as in the Carboniferous seas of 

 Scotland, and so far away from their present habitats ; although 

 it does appear to be somewhat strange that remains of true 

 Ch'itoneUi have not yet been detected in any strata of the several 

 formations that intervene between the Carboniferous and the 

 recent period. This organism seems never to have been abun- 

 dant, and the conditions under which it lived, in most cases in the 

 midst of coral reefs, may have been unfavourable to its after 

 preservation. The beds in Craigenglen in which these fossils are 

 found are shales alternating with a bed of Lithodendron jimceum 

 and L. faskulatum, corals which were the representatives of reef- 

 building forms during the Carboniferous limestone period, and in 

 the fissures of which the Chitonellus may have found a suitable 

 habitat in this early stage of its existence. Chitonellus difi'ers from 

 the true Chitons in the much smaller size of the plates, and in the 

 small portion of the plate exposed. This is well seen in Dr Thom- 

 son's specimen, which is about 3 inches in length and J of an inch 

 in diameter. The animal is slug-like, and has the small plates 

 inserted in the mantle at increasingly wider intervals as they 

 extend from the anterior to the posterior end, where they are 

 widest apart. The exposed portion of the plate is seen to be 

 rather less than \ of an inch in length, the inserted portion being 

 narrow, and having the apophyses close together. In these 

 characters the recent and fossil species agree very closely. For 

 the sake of comparison, Mr Young had mounted the plates of 

 another species, Chitonellus striatus. These were obtained from a 

 dried and much-shrivelled specimen in the Hunterian Museum, 

 and they likewise show how closely the recent and fossil species 

 are related to one another. Two species of Chitonellus have been 

 obtained from the shales of Craigenglen, which still continues, 



