470 TERMINOLOGY OF 



the other inferior, the superior overlying the dorsal, and the 

 inferior sustaining the ventral aspect of the animal's body.* 

 In a conchiferous Mollusk, on the contrary, the back looks to 

 the superior or cardinal margin, and the belly to the inferior 

 margin or gape. 



The shell of the Brachiopods is almost always regular 

 and symmetrical, viz. a line drawn longitudinally from the 

 summit to the middle of the inferior margin divides it into 

 four equal areas. 



The muscular impressions are 

 * '§• 89, more than two and faintly mark- 



ed. There are usually three or 

 four on each valve. The mantle 

 also adheres throughout very 

 firmly to the inner surface, and 

 leaves no marked impressions 

 (Fig. 89). 



When the valves are unequal, 

 as they mostly are in this order, 

 the largest valve is the dorsal 

 (Fig. 89). Its apex is more or 

 less elongated, and usually perforated. It is patelliform in 

 the Crania and Orbicula, and more or less pyramidal in 

 Calceola and some Terebratulae, in which the inferior or 

 ventral valve is reduced almost to be operculiform. 



The beaks of the valves, when equal, are always opposite 

 and look to each other ; when unequal, the beak of the dorsal 

 valve is the largest and most developed. 



The beak of the dorsal valve offers to our notice certain 

 modifications which have been used to distinguish groups in 

 the order. It is sometimes short and almost obsolete, but, 

 through a series of species, it is seen gradually to enlarge 

 and to project beyond the valve, sometimes in a pyramidal 

 shape, and sometimes more or less spiral, rounded entirely 

 or, not unfrequently, flattened on the side of the hinge. The 

 most important character, however, is its perforated* or entire 

 state. The perforation is either a hole or a deep sinus or notch. 

 (See Fig. 89). Usually the perforation is on the summit of 

 the beak, which is there observed to be truncated ; but it is 

 often found between the summit and the hinge. 



'er 



* " The perforated valve (Fig. 89) is the upper or dorsal one, while the 

 other is the lower or ventral (Fig. 90) ; this last being usually furnished 

 with an appendage, assuming various forms in different species, for the sup- 

 port of the parts of the body of the animal. When the shell is placed on 

 the lower valve with the hole or gap towards the observer, the sides of the 

 shell will correspond with his own." — J. E. Gray, in Zool. Journ. i. 209. 



