210 BRITISH MOLLUSKS. 



Senegal and Central American species, is conspicuously distin- 

 guished by its extremely gibbous trapezoidal form, and transverse 

 zigzag colouring. In young specimens, clustering in bunches and 

 in masses one upon another, the pretty waved pattern is particu- 

 larly bright and distinct. As the specimens advance in growth, 

 and the epidermis thickens and becomes fibrous, the pattern is 

 nearly obliterated. 



M. Moquin-Tandon and Mr. Jeffreys describe the animal as 

 being greyish white, yellowish or fawn-colour at the posterior side, 

 striped like the shell with zigzag marks of reddish brown. The 

 cirrhi of the branchial orifice, arranged in concentric rows, are 

 reddish grey, tinged at their base with brown, and the foot is grey 

 with a delicate rosy hue. 



Order II. BIMUSCULOSA— two-muscled. 



In this Order, which comprises the greater portion of the Ace- 

 phala, marine as well as freshwater, the shell is affixed to the 

 animal by a pair of adductor muscles, adhering to the valves of 

 the shell at each end. All have the lobes of the mantle more or 

 less united. We have in Britain two Families of freshwater two- 

 muscled bivalves, the Naiades, including the genera Anodonta and 

 TJnio, and the Cardiacea, including Pisidium and Cyclas. The 

 mantle lobes of the Naiads, the largest of all freshwater mollusks, 

 are freely open, except where they are united behind to form the 

 branchial and excretory siphonal orifices. In the Cockles of fresh 

 water, which are small, some of them minute, the mantle lobes are 

 more united on each side, and the branchial and excretory siphons 

 are prolonged into tubes. In Pisidium, the tubes are blended in 

 one, in Cyclas they are united for some distance and then separated. 

 The shell of the Naiades is composed of firm pearly matter, covered 

 by a thick fibrous epidermis ; the shell of the Cardiacea, that is to 

 say, of the freshwater genera of Cardiacea, is thin, covered by a 

 horny epidermis. 



Our two-muscled freshwater Acephala are comprised in two 

 Families :— 



1. Naiades. Animal bearing a solid pearly shell, with the mantle 

 lobes freely open, except behind, where they are united to 

 form the branchial and excretory siphonal orifices, which are 

 simply pouted. Foot large, free. 



