﻿T. Davidson — What is a Brachiopod? 201 



shall adopt, as tliey seem to carry with them a greater degree of 

 plausibility^ (PL X. Figs. 1, 2). Of the shell or valvular muscles ho 

 makes out five pairs and an odd one, and individualises their 

 respective functions as follows. Three pairs are laterals, having 

 their members limited to the sides of the shell : one pair are 

 transmedians, each member passing across the middle to the 

 reverse side of the shell : while the odd muscle occupies the um- 

 bonal cavity. The central and umhonai muscles effect the direct 

 opening and closing of the shell : the laterals enable the valves to 

 move forward and backward on each other : and the transmedians 

 allow the similar extremities (the rostral) of the valves to turn from 

 each other, to the right or the left, on an axis subcentrically situated, 

 that is, in the medio-transverse region of the dorsal valve. It was 

 long a matter in discussion whether the animal could displace its 

 valves sideways when about to open its shell ; but this point has 

 been set at rest by Professors Semper and Morse, who observed the 

 animal perform the operation. They mention that it is never done 

 suddenly, or by jerks : as the valves are at first always pushed to 

 one side several times, and back again on each other, at the same 

 time opening gradually in the transverse direction, till they rest 

 opposite to one another and widely apart. Those who had not seen 

 the animal in life, or who did not believe in the possibility of the 

 valves crossing each other with a slight obliquity, would not consent 

 to appropriating any of its muscles to that purpose, and consequently 

 attributed to all the lateral muscles the simple function of keeping 

 the valves in an opposite position, or holding them adjusted. We 

 have not only the observations of Semper and Morse, but the anato- 

 mical investigations of King to confirm the sliding action or lateral 

 divarication of the valves of Lingula. 



In the Clistenterata, where no such sliding action of the valves was 

 necessary, or possible, no muscles for such an object were required : 

 consequently none took rise from the lateral portions of the valves, as 

 in Lingula. In an extinct group, however, the TrimerellidcB, which 

 seems to be somewhat intermediate in character between the Treten- 

 terata and Clistenterata, certain scars have been found which appear 

 to have been produced by rudimentary lateral muscles ; but it is 

 doubtful (considering the shells are furnished with teeth, though but 

 rudely developed) that such muscles enabled the valves, as in Lin- 

 gula, to move forward and backwards upon each other. There are 

 muscles connected with other portions of the animal, such as the 

 parietal so strongly defined in the Tretenterata, and distinctive pecu- 

 liarities of the peduncle, etc.; but the limited space at my disposal 

 will not admit of more than a passing allusion to them.^ 



Banges of Depth. — All Brachiopoda are inhabitants of the sea. A 

 vast amount of important and accurate information has been collected 

 during the past ten years with respect to the geographical distri- 



' King, Annals and Mag. of Nat. Hist., 4tli series, vol. xii., 1873. 

 2 Davidson and King, on the Trimerellidce, Quarterly Journal of the Geological 

 Society, 1874, vol. ixx. p. 124. 



