ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
103 
lajer, their ends pointing towards the two extremities of the valve. This 
fusiform structure is very commonly larger or coarser in the external layer 
than in the internal, as will be seen in tigs. 16 and 17» wliioJi represent sec¬ 
tions of the two layers in both taken parallel to tiie sur/ace and in 
the same direction. The tubular structure occasionally presents itself in the 
shells of this family, but chiefly in the neighbourhood of any prolongations 
of (heir surface in ridges, points or foliations. 
XI. Mactrac€CB. 
77. Passing by the small family of Lithopbaga, which presents nothing 
remarkable as regards the intimate structure of the shell, we come to the 
extensive group of genera allied to Mactra. In most of these, the texture of 
the shell presents the same general characters as in the preceding family, 
but the indications of organic structure are more distinct. Thus, on the ex¬ 
ternal surface of dlarfru sfullorum (fig. 18) Is a layer whose ccHoIar cha¬ 
racter cannot be questioned, although the cells are irregular in form and 
somewhat indefinite as tu their boundaries; and in the inner layer of the 
Same shell the boundaries of the elongated cells are extremely distinct. In 
fig. 19 is shuwn a section of the internal layer of LutrarUi eUipiicoy which 
has passed someivliat obliquely to the elongated fusiform cells and shows 
their extremities cropping out, one set above another. In I^ryciitn cardioides 
the external layer contains a large amount of minute tubuli, corresponding 
with the sculptured character of its surface; and the fusiform cellular struc¬ 
ture is also very evident in some parts of this layer. In Aiiijihidestna, also, 
clear iudieatious of cellular structure may be discerned upon the surface. 
XII. Myacem. 
78. Uuiler this title I bring together (merely for the sake of convenience) 
the two Lamarckian fa«nilies, Myaires and Vorhul^.es ; since l)C‘lwceii Mya 
and Corbida thei’c is no clear line of demarcation. As will presently appear, 
Mwever, the structure of tlic shell in Pandora is so difl'erent from that of 
Myu and its cougeners, as fully to sanction the siiparation of that genus frmn 
die family, and to warrant its being regarded as the typo of a distinct group, 
hi all the shells of this family which 1 have examined,—including examples of 
from the genera Mya, Cvrbula, /inaiinuy Thruvia, Myothra and 
^^izodtmay —very distiiiet indications of cellular structure aro to he traced, 
iflese are of a particularly interesting nature in Mya arenarkh llie careful 
e^rainatioii of whose shell has thrown jutjcli light on several doubtful points 
niy inquiry. We have htTC a dUtiuctiy cellular structure in some parts 
20), giving way by such imperceptiblf gradations to an almost perfectly 
OBjogeneous arrangement in others (fig. 21), that no separation can be 
made between them; so that we must regard the. latter as having had tho 
origin with the former, although its primary character haw been lost. 
Mr the external surface of the sbel! is a layer of cells having distinct boun- 
ncs and large dark nuclear spots (fig. 22); and yet in other parts of that 
.. , , boinidaries of the cells are completely obliterated, and only 
nwk nuclear spots remain to show lliulr original divisions (tig. 23). In 
Bit instances the continuous cells seem to coalesce in sinuous rows, so that 
»«vy hue* are loft (somewhat resembling the bouudarie.s of the furrows of 
2ydr,m) dividing one series from miotlmr (fig. 2+). Kew external 
wnace some very large cells am disposed without any regularity, amongst 
made up (fig. 25); and the exU-rual sur- 
Itself 13 composed of small celb of rounded form, in by no means close 
