644 



MOLLUSCOIDEA. 



types of the Brachiopods, as in the genus Rhynckonella, the shell- 

 structure is simply fibrous. In many forms, however, 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, being 

 in general more or less dilated just before they reach their termina- 

 tion on the exterior of the shell (fig. 489, b and d). These canals 

 give the shell a "punctated" structure, and in the living animal 

 they contain csecal tubuli, or solid prolongations, derived from the 

 mantle. In some forms (as in Producta and Chonetes) the tubuli 

 do not reach the exterior of the shell, the outer layer being simply 

 fibrous. Though the shell is in some groups of the Brachiopods 

 always " punctate," there are nearly allied types in which the shell 

 may be "impunctate" in one case and "punctate" in another. 

 Thus, the shell is simply fibrous in Rhynchoiiella, but is tubulated 

 in the nearly related genus, or sub-genus, Rhynchoporina. 



The two valves of the shell in any Brachiopod are articulated 

 together by an apparatus of teeth and sockets, or are kept in apposi- 

 tion by muscular action alone. As regards the contained animal, 



Fig. 490. — Rhynchonella sulcata, a, Profile view; b, View of the dorsal surface ; c, View of the 

 base, a, Ventral valve ; b, Dorsal valve ; f, Base ; c, Beak ; k, Foramen. Lower Cretaceous. 



the position of the valves is anterior and posterior, so that they are 

 properly termed the " ventral " and " dorsal " valves. One of the 

 valves is always slightly, sometimes greatly, larger than the other, 

 so that the shell is said to be " inequivalve " (fig. 490). On the 

 other hand, a line drawn vertically from the beak of the shell to its 

 base (in fig. 490, b, from c to/) would divide it into two equal halves, 

 so that the shell is said to be "equilateral." In the true Bivalve 

 Shell-fish (Lamellibranchiatd), on the contrary, the valves of the 

 shell are placed upon the sides of the contained animal, so that 

 they are " right and " left," instead of being dorsal and ventral. 

 Further, the two valves are usually of the same size (" equivalve "), 

 and a line drawn from the beak to the base would almost always 

 divide the shell into unequal halves ; so that the shell is " inequi- 

 lateral." 



