10 



From tlicir inner side numerous secondary vessels {q) strike 

 inwardly, with a backward curve, and meet in the middle line 

 of the valves: others, larger than the latter {q), run off 

 from the outer side of the primaries in an oblique forward 

 direction, and parallel to one another ; but they become sud- 

 denly and simultaneously constricted, and next somewhat 

 rapidly attenuated, before reaching the sliell-margins. Both 

 the primaries and secondaries are slightly branched in the 

 middle of the anterior part of the brachiocoele. The constric- 

 tion of the outgoing secondaries forms a line {v) which runs 

 round the margins from one side to the other of the anterior 

 parietal : here the line is about an eighth of an inch from the 

 margins, which distance is gradually increased to a quarter of 

 an inch at the front. Both sets of secondary vessels give the 

 interior of the pallia! lobes a strongly plicated or ribbed 

 character. 



At its departm-e from the anterior parietal, each of the 

 primary vessels sends off direct to the margin one or more 

 branches, which, instead of stopping suddenly, like the outgoing 

 secondaries, are abruptly turned backward, passing along the 

 narrow space on the outside of the ante-latero-parietals, and 

 entering the pleurocoeles. These branches ^\i\\ be noticed 

 again shortly. 



The vessels of the brachiocoele, though prominent, do not 

 produce such strong impressions on the inner surface of the 

 valves as might be expected. Con-esponding impressions are 

 often more marked in recent Terebratulids, &c. ; and they are 

 frequently beautifully displayed in various species of extinct 

 genera. The sj^ecimen of Orthis striatula * originally be- 

 longing to Dr. de Koninck, and now in the British Museum, 

 and several other fossils that have been figured by Davidson, 

 show them very distinctly ; also Leptcena analoga f, and 

 Caviarophoria multiplicata\. In the last fossil the vascular 

 impressions even show a median line, which seems to corre- 

 spond with the mid rib inside the vessels in Lingida §. 



Pleurocoeles or side chambers. — Reverting to the main 



* Introduction to Fossil Brachiopoda, pi. vii. fig. 133. 



t Monograph of Permian Fossils, pi. xx. figs. 6 & 7. 



I Ibid. pi. viii. figs. 6 & 7. 



§ The mid rib is the septum of Semper, who has discovered that on 

 one side of it the blood flows forward, and on the other side backward. 

 Failing to detect any contractile organs, or the so-called '' hearts," and 

 from certain evidences, he is of opinion that the circulation of the blood 

 is effected by the action of cilia lining the inside of the vessels. Each of 

 the primary vessels appears to be divided at its origin. Assuming this 

 interpretation to be correct, and adopting Semper's opinion, one division 

 may serve for the outflowing, and the other for the inflowing current. 



