TEREBRATULA. 355 



the deltidia are almost obsolete. The internal appendage, 

 which is always attached to the lesser valve, is very small, 

 not extending more than one third the distance to the 

 ventral edge ; it is arched and vaulted below. There is 

 occasionally, in those individuals whose bases are the more 

 emarginated, an obscure indication in one or both valves 

 of a broad and shallow mesial groove. A fine individual 

 measured eleven lines from the apex to the base, and three 

 quarters of an inch from side to side. 



The arms or buccal appendages of the animal occupy 

 the greater part of the cavity of the shell. They are fixed 

 to and follow the course of the apophysary skeleton, and 

 appear, when the shell is forcibly opened, in the form of 

 a pair of brilliant orange or crimson fringed loops lodged 

 in each half of the cavity of the imperforate valve. The 

 outer margins of each loop bear long cirrhi also of a 

 brilliant orange or crimson hue, and though the arms 

 themselves cannot be protruded, these cirrhi are very 

 extensile ; when the animal is lively, the two valves 

 separate and gape for no very great distance from each 

 other in front, and from their sides are seen the long 

 crimson cirrhi extended like a pair of double fringes, and 

 borne somewhat stiffly and with a slight curve outwards. 

 Towards the edge of the strongly adherent mantle attached 

 to each valve, are placed at regular intervals about forty 

 small cirrhi of a softer texture, which do not appear to 

 be protruded, at least conspicuously, beyond the edges of 

 the shell. These cirrhi are tinged with crimson also. 

 At their bases are seen, when a high magnifying power 

 is used, coloured dots and cavities with vibrating cor- 

 puscles, which may be regarded as ocelli and otilitic cap- 

 sules. The whole surface of the mantle is studded with 

 vibratile cilia. On each side of the inner surface of the 



