Introduction to A n imal Morphology. 257 



be rudimentary, or the laterals or all may be absent (Mytilus). 

 The animal may lie permanently on one side, having its 

 smaller valve uppermost, and this side may be constant for 

 the species (Oyster, Pecten, &c.), or may vary (Chama). A 

 few are found in the morphological position with the foot 

 downwards, fixed (Humphreysia), or free (Lepton). Under 

 the periostracum in the shell is a layer of prismatic elon- 

 gated cells ; polygonal in section ; secreted by the mantle 

 edge ; deeper in the nacreous or mother of pearl layer ; 

 secreted by the mantle surface ; constantly increasing in 

 thickness during life. The prismatic layer is absent in 

 Cyclas, &c., and the membranous wall of the shell is there 

 pierced by many pore canals. The outer stratum of this 

 layer in other shells is pierced by many branching tubes (the 

 hyph;£ of a fungus, Kollikcr). Foreign bodies between the 

 mantle and the shell become coated with this shell layer, and 

 form pearls. A fine, white, structureless layer underlies the 

 nacre, consisting of calcareous granules in an organic base- 

 ment. The sizes of shells vary from i" to 2^-', and the 

 weights from 3 grains to 30olbs. The colors are white, red- 

 dish brown, green or olive (the last in freshwater forms). 



Irregularities of structure and arrangements are met with 

 in the shells of the extinct sub-class Endocardines, and in 

 the living Tubicola, where the animal inhabits a calcareous 

 tube, to which the valves may be solidified (Aspergillum), or 

 the right solidified and the left free (Clavagella). In these 

 the hinge ligament is at first flexible, but becomes calcified. 

 In Anatinidae a calcareous nodule develops in the hinge carti- 

 lage.*' Shells may have flat processes on each side of the 

 umbo {auricles, as in Pecten, or, when large, alae, as in Perna 

 Avicula). In Spondylus the teeth are locked, limiting the 

 motion of the valves. In Pholas, beside the ordinary lateral 

 valves, there are other accessory shell plates, two umbonal 

 plates, a dors-umbonal, and a dorsal in place of the ligament. 

 The shell structure in the teeth of many forms is radiate, 

 appearing in sections like the mineral Wavellite. Shell con- 



* In Venus, Cardium, &c., tlie cartilage is very small, in a cavity be- 

 tween the primary teeth. 



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