J.17 



MONOGRAPH OF THE GENUS 

 SPONDYLUS. 



Clms, Conchilera. Order, Monomyaria. Fam. Pectinides. 



Gen. Char. Testa insequalvis, auriculata, irregulariter costata et 

 foliacea ; valva inferior! affixa, area cardinali triangulari ligamenti- 

 fera; cartilagine resiliendi in foveam arese cardinalis disposita; 

 dentibus duo validis in utrque valva. 



The SpondijU are inaequivalve, auriculated, irregularly foliaceous, 

 ribbed, and generally spinous or foKaceous, The lower valve, 

 which has an external, triangular area, is attached to sub-marine 

 substances by a greater or less portion of its surface. The elastic 

 cartilage of the liinge is placed in a linear grove in the centre of 

 the cardinal area, which terminates in a rounded spoon-shaped 

 process within. There are two teeth, locking into corresponding 

 cavities in each valve. 



The younger portion of the shell generally takes its form from 

 the substance to which it is attached, and it is not until the lower 

 valve in its growth parts from that substance that the true shape 

 can be known. If this takes place at an early period, the shell is 

 regular, and generally of an obhquely oval form ; but if, as in 

 some cases, the whole lower surface is in contact with the rock or 

 madrepore, it is liable to every kind of variation. In some species 

 there appears to be a tendency to grow more freely than in others, 

 as in S. regius, S. Cmningii, and more particularly in S. imperial'is, 

 in which the point of attaclunent is scarcely perceptible. The 

 spines, or scales, or imbrications, are also much influenced by the 

 circumstances and position in which the growth has taken place, 

 many shells being spinous at one period of their development, and 

 foliaceous at anotlier. In the lower valve, where the surface ceases 

 its contact with the rock, there are frequently tlu-own out broad 

 foliations, apparently for the purpose of reaching it, and rendering 

 the position of the mollusc more firmly secure. These variations 

 are so perplexing and complicated, that it is quite impossible, ■\\'itli 

 any degree of confidence to define the species, or to fix their limits 



