104 



Adhering to shells or stones, and modifying the form and 

 surface of their shells in accordance with the inequalities 

 of their place of attachment ; thus, the same species will be 

 convex if on the outside of a Natica, concave if on its inte- 

 rior wall, ribbed in either direction, according to its attachment 

 on the exterior of a Pecten, etc,; others again are attached in 

 groups one over another on each others' shell. 



The distribution is world-wide ; the individual species have a 

 wide range, which, added to their great variability, as in 

 attached shells generally, has caused an enormous specific 

 synonymy. Genetically, the synonyms include Sandalium, 

 Schum., 1817, Crypta, Humphrey, 1797, Tylacus and Lyro- 

 scapha, Conrad, Crepipatella, Lesson, 1830, Proscenula, Perry, 

 1811. 



The following sections are founded on characters of little 

 importance or stability : 



Section G-ARNOTIA, Gray, 1867. 



Apex median, posterior, detached and distant from the 

 peritreme. 



Section CRYPTA (Gray, in part, 1867), Fischer, 1885. 



Surface spinose. 



Section IANACUS, Morch, 1852. 



Shell depressed or concave above, apex posterior, but 

 slightly lateral. 



Section SIPHOPATELLA, Lesson, 1830. 



Lamina produced in front, its columellar margin subtubular. 

 Ergxa, H. and A. Adams, 1854, and Noicia, Gray, 1867, are 

 synonyms. 



Section SPIROCRYPTA, Gabb, 1864. 



Summit of shell posterior, lateral, submarginal, spiral ; 

 internal plate attached to the margin on the lower or outer 

 side, curving upwards and inwards, and uniting with the oppo- 

 site side at a considerable distance. The plate is subspiral, 

 thus approaching Infundibulum and Galerus. G. pileum, 

 Gabb (Struct, and Syst. Conch., t. Ixiv, f. 78). 



Cretaceous of California. 



