Mr. A. Hancock on the Organization of the Brachiopoda. 143 



the dorsal adjuster muscles are not attached to a hinge-plate, but are 

 inserted into the valve itself. In Rhynchonella psittacea there is a 

 pair of peduncular muscles. In Lingula there are six pairs of muscles, 

 all of which have both extremities attached to the valves. They have 

 been divided into adductors and sliding muscles, the latter again being 

 subdivided into protractors and retractors ; but the author, con- 

 sidering that no sliding motion takes place, regards the latter terms 

 as improper, and gives a set of new names, of which a concordance 

 with the older denominations is given on the preceding page. 



The author conceives that the valves are separated by the action 

 of the divaricators, combined with that of the parietals ; these 

 muscles compressing the visceral cavity posteriorly, and thus driving 

 its contents into the anterior portion. The antagonists of these are 

 the occlusors ; while the office of the adjusters appears rather to be 

 to supply the place of a hinge, and to prevent anything like sliding 

 of the valves one over the other. 



The muscular fibres of Lingula are smooth and unstriated. In 

 Waldheimia those of the posterior occlusors are strongly striated, but 

 the rest of the muscles have smooth fibres. The arms, their attach- 

 ment and minute structure are next fully described. 



In Waldheimia the canals of the attached portions of the arms 

 coalesce into a single wide tube, which lies externally between the 

 produced and reflected crura of the calcareous loop, and is separated 

 by a partition from a canal of corresponding size — the ** brachial 

 sinus," — which also extends throughout the whole length of the pro- 

 duced and reflected crura, and is in fact a prolongation of the peri- 

 visceral chamber. The cirri are arranged in this and all the other 

 Brachiopoda examined, in a double alternating series — not in a single 

 row, as has hitherto been stated to be the case. The walls of the 

 brachial canal are tolerably well supplied with delicate muscular 

 fibres, which run diagonally round the tube, and are most strongly 

 developed towards the sides, near the grooved ridge which supports 

 the cirri. An indistinct band of exceedingly delicate longitudinal 

 fibres may also be observed nearly opposite to it. The author has 

 however completely failed to discover, either here or in Rhynchonellay 

 anything like the double spiral arrangements of fibres described by 

 Prof. Owen, and believes that the latter observer has mistaken the 

 blood-sinuses for muscles. 



The author doubts whether the spiral coil can be unwound, and 

 conceives that the muscular fibres described, are chiefly for the pur- 

 pose of giving firm support to the grooved ridge on which the cirri 

 and brachial fold are seated, and thus affording the complex mus- 

 cular fibres which the ridge contains a better fulcrum whence to act 

 upon the cirri. 



In Terebratulina caput-serpentis, which possesses no calcareous 

 loop, the pallial lobe connecting the produced and reflected portions 

 of the arms is strengthened by calcareous spicula, which are so 

 numerous as to preserve the shape of the part even when the animal 

 basis is removed. 



In Lingula the arms contain two canals ; one, the anterior, being 

 the equivalent of the single canal in Rhynchonella^ and, like it, ter- 



