DE. T. DAVIDSOX 0^' EECEXT BEACHIOPODA. 203 



centric linos of growth, tlic riblets increasing in number botli by bifurcation and by 

 the intcr2:)olation of sliorter ones. In the interior of the upper or lui'ger valve four 

 adductor muscular scars limit posteriorly and anteriorly a rather large sunken space, 

 which occupies, as it does in the lower valve, about half the posterior half of the valve. 

 The posterior adductor muscular impressions are transversely oval, oblic^uely situated at 

 a short distance from th(^ posterior margin of the shell, and separated by two very small 

 impressions or j^jrojections attributed to retractor muscles. The anterior, oval-shaped, 

 adductor scars are situate towards the middle of the bottom of the shell, and arc obliquely 

 placed and separated by a small longitudinal ridge, which extends for a short distance 

 along the sunken portion of the valve. In the interior of the extremely thin lower or 

 smaller valve its posterior half is, to a great extent, occupied by a heart-shaped disk with 

 convex lateral borders, but which does not quite reach to the jiosterior edge of the shell. 

 Along part of its centre a small, longitudinal, oval-shaped foramen is seen ; two small, 

 oblique, adductor scars occupy the posterior edge of the disk, and two others the anterior 

 part of the same disk, these last being separated by a longitudinal ridge which, com- 

 mencing to rise at the anterior extremity of the foramen, extends nearly to the front 

 margin of the shell. Shell-structure horny and calcareous. Colour light brownish yellow. 

 Length 9 lines, breadth 8 lines. 



Hab. Cape St. Lucas to Panama, Central America ; dredged at Payta, St. Elena, Island 

 of Cafia, Guatemala, attached to the lower side of stones in sandy mud at low water, 

 and in some instances from a depth of from 6 to 8 fathoms (Cuming). Ecuador (d'Or- 

 bigny) ; Mazatlan, on various shells (Carpenter). Isle St. Joseph, Cayenne (Deplanche). 



Obs. Exteriorly both in shape and striation the upper valve of Discinisca Cuni'mgii 

 much resembles that of Disclna striata ; but the smaller one, flat or concave in the 

 first, is convex in the latter. Again, the absence of the flattened disk in D. striata 

 and the difference in the shajie and position of the foraminal aperture are characters by 

 which the two forms may be easily distinguished. 



In his monograph on Orhicula Mr. L. Peeve observes that "Mr. Broderip's 0. strigata, 

 which he did not describe along with 0. Cumingii in the ' Proceedings of the Zoological 

 Society,' but as an afterthought, when figuring the Orhiculce in the ' Transactions,' is a 

 less worn state of the species, in which there are faint rays and bands of colour. The 

 upper valve is calcareous and firm, of quite a different type from the horny species of 

 Chili and Peru. The habitats, Malacca and Philippine Islands, given with this species by 

 Mr. Sowerby, in addition to the above, are erroneous. He probably mistook specimens 

 of 0. Stella for it." 



Broderip's figures of Orhicitla Cumingii and 0. strigata differ a good deal in shape; for 

 while the posterior margin of the first is I'ounded, that of the latter is strongly indented ; 

 but this may be accidental, lie likewise represents the mantle-fringe of horny setae in 

 his figure of Orbicula Cumingii ; they are very much shorter in this species than in 

 Discinisca atlantica. 



Discinisca Cumingii varies a good deal in the elevation of its upper valve ; in some 

 specimens the apex is also almost marginal, while in others it is more or less distant 

 from the posterior margin. 



