198 DE. T, DAVIDSON ON EECENT BEACHIOPODA. 



elevated, convex, oval-shaped area, with a small, sharp, central, triangular-shaped promi- 

 nence in front of the foramen, which separates the anterior pair of adductor muscular scars. 

 The fissure, which is narrow, begins at a short distance from the posterior margin, and 

 extends to about two thirds of the length of the disk. The animal is extremely delicate 

 and transparent ; mantle-lobes distinct all round and not adliering to the interior of 

 the shell, which is smooth and polished, highly vascular, bordered with a dense fringe 

 of long horny setae, which are stiff, barbed, and extremely brittle. Labial folds united, 

 not extensile. Spiral extremities of the arms directed towards the lower valve, and not 

 dorsally, as in Crania. Length and breadth nearly 1 inch. 



Sab. Iquiqui, adhering to living Mi/iilus, and Bay of Ancon, Peru. From Panama ta 

 Peru (Cuming). It occurs in groups or piled one over the other in vast numbers on 

 sandy bottoms and at depths of from 5 to 9 fathoms. Mr. Reeve mentions that at Ancon 

 Discmisca lamellosa was found attached to dead shells, also clinging to the wreck of a 

 Spanish vessel of about three hundred tons that went down in the bay about ten years 

 before. A. d'Orbigny states that it is common on all the shores of the Great Ocean, from 

 Cobija (Bolivia) to Callao (Peru), and that be has obtained it in shallow sea in great 

 nu.mbers near the island of San Lorenzo. 



Ohs. This species has been well figured by Broderip, Sowerby, L. Beeve, and others. 

 The animal was anatomically described and illustrated by Owen in the Trans, of the Zool. 

 Soc. of London, vol. i. 1835, as well as by Dr. S. P. Woodward and myself in 1852 *. 



Prof. Owen states (p. 153, loc.cit.): — " On carefully removing the imperforate valve, 

 the vascular mantle is seen with the margin entire in the whole of its circumference. 

 The muscles and viscera form a rounded mass, situated in the posterior half of the shell. 

 First are seen the extremities of two muscles, of an oblong figure, converging ante- 

 riorly, and measuring two lines by nearly one : in the triangular space between these 

 muscles is situated the green liver, behind which is the grey ovary ; and at the poste- 

 rior part of the circle are the extremities of two smaller muscles. The four impressions 

 of these muscles are observable on the interior of the shelly valve. 



"On removing the lower valve, which should be cut through from either side as far as 

 the fissure in order to avoid disturbing the soft parts, the vascular lobe of the mantle 

 with similar free margins is exposed, but the viscera are quite concealed by the dilated 

 disk or foot. 



" Each lobe of the mantle can be reflected from before backwards to the extent of five 

 lines, and from behind forward to the extent of half a line, but they adhere too closely 

 to the visceral mass to be detached without laceration. When so reflected, the branchial 

 vessels may be seen in rich profusion on their inner surface. 



* [More recently Dr. L. Joubin has described (loc. cit. p. 33) the mantle of Dtsdiui as a thin membrane, closely 

 adherent, with a thickly ciliated border. It acts as a respiratory organ, bounds the visceral cavity, and contains 

 arborescent prolongations of the body-cavity as in Crania. The main vascular trunks are subdivided into branches 

 having numerous little orifices at their terminations. There is no trace of a heart or arterial system. The genital 

 o-lands resemble those of Crania and Lingula. The arms cannot be extruded beyond the shcU-margins ; the cirri are 

 thick and very long. The peduncle, which diifers much from that of Lin<jula, is formed of a fold of the mantle ; and, 

 being enclosed as in a sac, is entirely separated from the body-cavity. This structure is identical with that subse- 

 quently sliown by the same author (BuU. Soc. Zool. do Franco, vol. sii. 1SS7, pp. ll'J-126) to exist in young forms 

 of the articulated genera Ar<jiope and Terehratidhia. — A. C] 



