1 70 MYAD^. 



touch each other from the centre of the shell to its hinder 

 termination : the posterior dorsal margins bend outwards.* 

 The hinge consists of a remarkably large, solid, and erect 

 tooth in the left valve, and an appressed subtriangular and 

 oblique excavation under the umbones of the opposite 



* Our friend Dr. Carpenter, whose researches into the microscopic structure of 

 shells rank among the most important of recent contributions to Malacology, has 

 examined with great care the structure of this species, and gives the following 

 account of it in his forthcoming report : " The indications of cellular structure are of 

 a peculiarly interesting nature in ]\[ya arenaria, the careful examination of whose 

 shell has thrown much light on several doubtful points of my inquiry. We have 

 here a distinct cellular structure in some parts giving way by such imperceptible 

 gradations to an almost perfect!}- homogeneous arrangement in others, that no 

 separation can be made between them, so that we must regard the latter as 

 having had the same origin with the formei-, although its primary characteristic 

 has been lost. Near the external surface of the shell is a layer of cells, having 

 very distinct boundaries and large dark nuclear spots ; and yet in other parts 

 of the same layer the boundaries of the cells are completely obliterated, and only 

 the dark nuclear spots remain to shew their original divisions. In some instances 

 the continuous cells seem to coalesce in sinuous rows, so that wavy lines are left 

 (somewhat resembling the boundaries of the furrows of Meandrina) dividing one 

 series from another (fig. 24).* Near the external surface some very large cells 

 are disposed without any regularity, amongst those of which the layer is chiefly 

 made up (fig. 25) ; and the external surface itself is composed of small cells of 

 rounded form, in by no means close approximation with each other. In the 

 tooth, also, we find a considerable variety of structure, in addition to those forms 

 presented by the shell. Thus, in fig. 26, is seen a group of large cells, the cal- 

 careous contents of which are disposed in a very regular radiating Aragonite or 

 Wavellite. The borders of this group pass into another cluster of cells (fig. 27) 

 that presents no trace of this curious structure (of which, however, there are some 

 indications in shell) ; whilst the latter gradually passes, by the obliteration of 

 its cell-boundai-ies, into a layer of very homogeneous aspect. Besides these, 

 there are several curious forms of elongated cells, some of them with square ter- 

 minations, as in fig. 28, and some pointed or fusiform, as in fig. 29. In these 

 last may be seen tranverse striae, closely resembling those of the long prismatic 

 cells of Pinna, and probably due to the same cause, namely, the spaces between 

 the striae which indicate their lines of junction. Upon this last circumstance I am 

 disposed to lay much stress, as indicating the really compound nature of the long 

 fusifonn cells, of which we have already seen some examples, but which are pecu- 

 liarly characteristic of the univalve group. Neither in the shells nor the tooth 

 of Mya arcnaria is there animal matter enough to give anything more than a 

 delicate membranous residuum, in which no vestige of cell walls can be traced." 



■' The figures refer to Dr. Carpenter's plate. 



