2ss GENERAL SUMMARY TO 



The Shell. 



All Brachiopoda are inhabitants of the sea, and vary extremely in shape and 

 character. The animal, or its soft parts, in every Brachiopod is protected by a shell 

 composed of two distinct valves, and these valves are essentially symmetrical or equal- 

 sided, except in cases of malformation. They are not, however, equal-valved, one of the 

 valves being usually deeper than the other. The larger number of Clistenterata [Tere- 

 bratula, &c.) have their valves articulated by the means of two curved teeth developed 

 from the margin of the ventral valve, which fit into corresponding sockets in the dorsal 

 one, or are unarticulated as in the Tretenterata {Lingula, Crania, &c), the valves being 

 maintained in position by muscular action. The outer surface of some species is smooth, 

 in others variously ribbed, sculptured, and ornamented in different ways, and in many 

 cases coloured with various shades of red, yellow, and bluish-black, and stripes or spots of 

 red and white. Traces of the original colour have been sometimes noticed among the fossil 

 species, as will be found recorded in the pages of this Monograph. As collecting has 

 progressed since 160G up to 1884, an enormous number of so-termed species have 

 been assembled and carefully studied; and so great is the modification in shape in the 

 same species and in the larger number of forms that one is continually at a loss to draw 

 even a conventional line between two closely-allied so-called species. When one deals 

 with large numbers of specimens the difficulty seems to increase. The shell in some 

 species is extremely thin, semi-transparent, and glassy, and especially so in very deep- 

 sea or abyssal forms, such as in Terebratulina Dallii, Dav., Terebratula Wgvillii, Dav., 

 Waldheimia tenera, Jeffreys, Discina atlantica, King, &c. In others the shell is thick 

 and sometimes massive. No Brachiopod discovered that has come under my notice has 

 exceeded a foot in breadth by something less in length (Produdus giganteus), and while 

 the space left between the valves for the soft parts of the animal is, in a large proportion 

 of the species, great, in others it is so small that one is at a loss to understand where the 

 animal could have been located. 



As remarked by Dr. Van Bemmelen, the growth of the shell, in thickness as well as 

 in extent, is exclusively the effect of apposition. The inequality in the thickness of the 

 valves in some forms is very remarkable. As already observed in my Carboniferous 

 Monograph the ventral valve of some specimens of Produdus Llangollensis attained 

 nearly one inch in thickness, while the thickness of the dorsal valve did not exceed a 

 couple of lines. In Thecidea leptoznoidea and Th. Mayalis, Desl., the ventral valve is very 

 large, whilst the dorsal valve is not much more than half the size of the ventral one, which 

 it only partially covers. 



Name of the valves. — The valves have unfortunately, like the muscles and other parts 

 of the animal, been described by different names, but that of dorsal for the smaller or 

 socket valve, and that of ventral for the larger or dental valve, are now in general use, 



