290 Mr. A. Hancock on the Animal of Myochama anomioides. 



in a very similar manner, there being perhaps some slight, but 

 unimportant, modifications. In Mz/ocAama, however, the branchial 

 and anal chambers are not perfectly divided as they are in Coch- 

 lodesma, and probably in Pholadomya. In the former the sep- 

 tum which cuts off the communication does so only partially, 

 there being a considerable aperture in it (tig. 1/) just where it 

 joins the extremity of the gills. It is quite possible that this 

 aperture may be the result of injury, for these parts are so ex- 

 ceedingly delicate that the examination of more than one speci- 

 men is necessary to determine this point with certainty. 



It is pretty clear too that the gills of this animal resemble 

 very closely those of Chamostrea ; the most important difference 

 being, that in the latter the dorsal border of the rudimentary 

 gill is free, while we have seen that in Myochama it is attached. 

 The minute structure of the organ is the same in both ; it is 

 therefore only necessary to refer to my paper on Chamostrea re- 

 cently published in the ' Annals,' where a detailed description of 

 this part of the subject will be found. It may be observed, how- 

 ever, that the surface of the branchial leaflet of the animal under 

 description is transversely plicated ; but the plicae ai-e not so nu- 

 merous, neither are they so delicate nor so much produced as 

 they are in that genus ; and it may be further remarked, that in 

 this respect Myochama agrees better with Cochlodesma, in which 

 the plicae are rather coarse and thick. Such slight differences 

 are certainly of very little importance, and can in no way affect 

 the function of the parts. The food will be secured on the sur- 

 face of the gills in Myochama exactly as in Chamostrea, and the 

 water will be strained through the organ in a similar manner ; 

 in the former as in the latter it will find its way, through the 

 vascular network forming the plicse, into the spaces or tubes be- 

 tween the gill-laminae, and from thence into the dorsal channels 

 leading into the anal or exhalant chamber, and so to the siphonal 

 outlet. From the I'udimentary gill the water will be strained 

 into the channel situated below it, and thus reaching the anal 

 chamber will pass out with the general current. 



In concluding these few remarks on the branchial organ, it 

 may perhaps be worthy of notice that there appear to be three 

 distinct modifications of gill-structure in the Lamellibranchiata. 

 In the first the laminae forming the gill-plate are composed of 

 filaments, either free or only slightly united to each other at 

 distant intervals, as in Anomia and Mytilus ; in the second they 

 are formed of a simple vascular network, as in Mya, Pholas, &c. ; 

 and in the third the laminae of the gill-plate are complicated by 

 the addition of transverse plicae composed of a minute reticu- 

 lation of vessels, as in Chamostrea, Myochama, Cochlodesma, &c. 



