Introduction to A nimal 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, Pectcn, &c.), or may vary (Chama). A 

 few are found in the morphological position with the foot 

 downwards, fixed (Humphrcysia), 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 

 1 by many pore canals. The outer stratum of this 

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

 hyplue 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 

 1% of calcareous granules in an organic base- 

 ment. Ti. of shells vary from i" to 2^', and the 

 ats from 3 grains to 3oolbs. The colors are white, red- 

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



Irn 3 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 

 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 Anatiniihu a calcareous nodule develops in the hinge carti- 

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

 umbo (auri'l-?, as in Pecten, or, when large, atae, as in IVrna 

 Avi< ul.'i). In Spondylus the teelh are locked, limiting the 

 motion of the vahfl i. In Pholas, beside the ordinary Literal 

 accessory shell plates, two umbonal 



i . a dors-umbonal, and a dorsal in place of tin- ligament. 



iicll structure in the teeth of many forms is ra-liate, 



<;tions like the mineral Wavellite. Shell con- 



* In Venus, Cardium, &c., the cartilage i ,ty be- 



tween the th. 



S 



