THE BRITISH FOSSIL BRACHIOPODA. 



323 



additional researches have been made by several zoologists, and, I believe, we are at 

 present fairly well acquainted with the characters and functions of this important part of 

 the animal. 



As the number and position of the muscles differ materially in the two great divisions 

 Clistenterata and Tretenterata into which the Brachiopoda have been grouped, and to 

 some extent also in the genera of which each division is composed, it may be desirable 

 to treat the subject under two separate heads. Unfortunately, almost every anatomist 

 who has written on the muscles of the Brachiopoda has proposed different names for 

 each muscle, and the confusion thence arising is much to be regretted. In the Clistenterata, 

 of which the genus Terebratula may be taken as an example, five or six pairs of muscles 

 are stated by Hancock, Gratiolet, and others to be connected with the opening and 

 closing of the valves, or to their attachment to, or movements upon, the peduncle. 



Fig. B. 



Waldheimia Jlavescens. 



Fig. A. Interior of the ventral valve. /, foramen ; d, deltidium ; t, teeth ; a, adductor impressions 

 ( = occlusors of Hancock); c, divaricators (=* cardinal muscles of King = muscles diducteurs principaus 

 of Gratiolet) ; c', accessory divaricators ( = muscles diducteurs of Gratiolet); b, ventral adjusters ( = 

 ventral peduncular muscles, or muscles du pedoncule, paire superieure, Gratiolet) ; V, peduncular 

 muscle. 



Fig. B. Interior of dorsal valve, c, c', cardinal process ; b, V, hinge-plate ; s, dental sockets ; I, 

 loop; q, crura; a, a, adductor impressions ; c, accessory divaricator ; b, peduncular muscles ; s 



First of all, the adductors or occlusors consist of two muscles, which, bifurcating 

 near the centre of the cavity, produce a large quadrangular impression on the internal 

 surface of the dorsal valve (Fig. B a, a), and a single divided one towards the centre of 

 the large or ventral valve (Fig. A a). The function of this pair of muscles is the closing 

 of the valves. Gratiolet, who has described with great minuteness the muscles of 

 the Brachiopoda, informs us that those which close and open the valves were the only 

 ones known to Pallas, and he defined their position and functions clearly. 1 The same was 



1 ' Miscellanea Zoologica,' 1766. 



43 



