544 LECTURE xxn. 



culum sometimes varies in structure in the same genus, being horny 

 in some species, and shelly in others of Ampullaria and Natica, — as 

 it is present in some volutes, cones, mitres, and olives, and absent in 

 other species of those genera, — and as some genera in a natural 

 family, e.g. Harpce and Dolium among the Buccinoids, are without an 

 operculum, whilst the other genera of the same family possess that 

 appendage, — it obviously affords characters of secondary importance 

 in classification. In Lithedaphus {Calyptrcea) equestris the whole 

 base of the foot secretes a calcareous plate which is cemented to the 

 rock, and the shell appears to consist of two valves. In the Chiton 

 the shell is divided into eight symmetrical pieces arranged like scales 

 upon the back : the first of these is the smallest, the last the longest 

 and most approaching the circular form. 



In the interior of the shell the muscular impression is usually 

 crescentic, with the horns turned towards the head of the animal. 



Most univalve shells are composed of three strata, which differ 

 in the arrangement of the calcareous particles : the innermost layer 

 is nacreous in the fucivorous univales, e. g. Patella, Haliotis, and 

 resembles enamel in the marine species. In Cassis rufa each 

 layer is composed of many laminas, which are perpendicular to the 

 plane of the main layer ; and each lamina consists of a series of 

 prismatic cells, adherent by their long sides : the laminae of the 

 outer and inner sides are parallel to the lines of growth, while those 

 of the middle layer are at right angles to them. In the Cowries 

 (^Cyprcece), there is an additional layer, which is nacreous, and formed 

 by the over -lapping mantle-lobes when the animal has attained its 

 full growth. Such shells are called " Cameo-shells," those ornaments 

 being formed by the removal of one layer, and the carving of the 

 next. Hunter* discovered that the molluscous inhabitant of a shell 

 had the power of absorbing part of its dwelling. Tliis property, 

 which is now generally recognised, is well illustrated by the thinning 

 of the parietes of the internal whorls of the Cones and Olives, from 

 which two out of the three layers of which they were originally 

 composed may be observed to have been removed. The absorption 

 of shell is also illustrated by the removal or smoothing down of the 

 spines of the Murices, as the growing whorl expands and overlaps its 

 predecessor ; by the flattening of the inner lip of the mouth of the 

 PurpurcB ; by the widening of the fascal aperture of the Fissurellce ; 

 and it gives rise to various other modifications in the form and 

 structure of shell in the progress of growth. Another change of 

 form is due to the physical decomposition or destruction of a part of 



* PhUos. Trans. 1 785, p. 343. 



