438 



MOLLUSCA. 



ous ; in other cases it is very narrow, or even does not exist. 

 In front of the foramen of the ventral valve, and very often 

 forming part of its circumference, there is commonly a tri- 

 angular plate, which may be composed of 'one or two pieces, 

 and which is termed the " deltidium " (fig. 276, m). In 

 other cases this structure is altogether wanting. 



Fig. 276. Terebratella Astieriana Cretaceous, e, Hinge-area ; m, Deltidium. 



In intimate structure, the shell of most of the Brachiopoda 

 (fig. 277) consists "of flattened prisms, of considerable length, 

 arranged parallel to one another with 

 great regularity, and at a very acute 

 angle usually only about 10 or 12 

 with the surfaces of the shell." 

 (Carpenter.) In most cases, also, the 

 shell is perforated by a series of minute 

 canals, which pass from one surface 

 of the shell to the other, in a more or 

 less vertical direction, usually widening 

 as they approach the external surface. 

 These canals give the shell a " punc- 

 tated" structure, and in the living animal they contain 

 csecal tubuli, or prolongations, from the mantle, which are 

 considered by Huxley as analogous to the vascular pro- 

 cesses by which in many Ascidians the muscular tunic, 

 or " mantle," is attached to the outer tunic, or " test." In 

 some of the Brachiopoda (as in the Ehynclionellidce) the shell 

 is " impunctate," or is devoid of this singular canal system. 



The inner surface of the valves of the shell is lined by 

 expansions of the integument which secrete the shell, and 



Fig. 277. Minute structure 

 of the shell of Terebratula, 

 showing the flattened prisms 

 of the shell, and the canals. 



