6 MOLLUSCA FROM THE CRAG. 



while the Bridlington bed was more purely marine, with a bottom apparently of sandy 

 mud, similar to what is exhibited by the newly discovered tranquil deposit resting 

 upon the Red Crag at Chillesford, where the water may have been of some con- 

 siderable depth. 



In estimating the dimensions of the shell in the following descriptions, the pro- 

 portions are given only as an approximation ; in most species, these are more or less 

 variable. The length is taken from the anterior edge of the shell to the outermost 

 portion of the posterior side, that being considered as anterior where the foot is pro- 

 truded, while the position of the ligament and the siphonal tubes, where they exist or 

 their presence shown in the shell by the sinuated form of the pallial impression, is on 

 the posterior side. Presuming, therefore, the animal to move with the foot foremost, 

 it will have its dorsal or hinge-part of the shell uppermost, and the diameter from the 

 umbo to the ventral margin is called its height, while the depth is measured from the 

 most tumid part of one valve to the corresponding place in the other. 



Anomia.* Linn. 1767. 



Anomia. Mailer. 1776. 



Glycimeris. Browne, 1756. 



Lampades (sp.). Gevers. 1787, fide Gray. 



Fenestella. Bolton. 1798, fide Herrmansen. 



Echion and Echioderma. Poll. 1791. 



Cepa. Humphries, 1797. 



Anomya. Agass. 1839. 



Generic Character. Shell irregular, inequivalved, subequilateral, ovate or sub- 

 orbicular, and fixed : lower or inferior valve more or less flattened, with a large 

 foramen or perforation, through which passes a bony or calcareous appendage for the 

 attachment of the animal ; upper valve, convex smooth or irregularly laminated, 

 sometimes striated, costated or muricated, often assimilating the body of the shell to 

 that on which it is fixed ; one muscular impression in the lower or fixed valve, with 

 three in the upper or convex one ; ligament internal, placed a little within the umbo of 

 the upper or larger valve, in a somewhat triangular pit, with a projection near the 

 edge of the foramen in the opposite valve, to which it is attached ; hinge Avithout 

 teeth. 



The animal of this genus, is said to have the edges of the mantle disconnected, 

 the margins bearing a double fringe of short scirrhous appendages, without ocelli or 

 rudimentary eyes. No siphonal tubes, and foot very small, nearly obsolete. The 

 adductor muscle is divided into three parts, making three distinct impressions on the 



* Etym. 'Avofiotos, unlike or unequal. 



