OliGANISMS m A(!ADIA. 1S3 



As viewed from above, this valve presents a hollow more or less obvious iu the front of 

 the visceral cavity. This hollow is sometimes a deeper depression, and corresponds to the 

 outside of a tubercle within the shell, which continued throughout the life of the occupant 

 to be a marked prominence of the interior of the valve, and forms the initial point of the 

 mesian ridge of the shell. The longest diameter of this hollow is about equal to one-half 

 of the length of the embryonic shell, and in some examples it contains four little pits, 

 which appear to mark the points of attachment of muscles (fig. Id). The two lateral pits 

 appear to answer to the anterior adductors, and the posterior pit to the anterior retractor. 

 The anterior adjustors in this stage of growth seem to have been at the margin of the shell, 

 and outside of the large depression above referred to, for a series of pits can be traced on 

 the adult shell from the margin of the embryonic shell well out toward the outer edge of 

 the valve (fig. 1/.) The posterior adjustors are jirobably indicated by a depression on 

 each side of the umbo. 



The space occupied at this time by the visceral cavity was large iu proportion to the 

 size of the shell, and extended quite out to the margin ; and no indication of the existence 

 of a marginal area for the protection of a mantle and sette can be seen. The outer posterior 

 angles of the embryonic shell were turned upward, giving it somewhat of a saddle-shaped 

 relief (fig. Id.) In some examples the shell is crossed by a raised baud due to the strength- 

 ening of the hinge line during the subsequent growth of the shell (fig. la.) 



Already, at this very early period, we find clearly, though very minutely displayed, 

 the rasp-like surface, which is so marked a feature of this species during the next period 

 of its growth. 



(2.) In the sfecond phase of growth there is a decided change in the form of the shell. 

 There was also an extension of the hinge line, and a transfer outward of the muscles along 

 that line to accommodate the growth of the animal ; but although the hinge line is 

 actually much longer than in the embryonic shell, owing to the more rapid enlargement 

 of the sides and front it appears to be shorter. This we may regard as the larval stage 

 of the shell, as by its form and features it exhibits indications of the possession of organs 

 for the capture of food (fig. 1/.) 



The overlapping outer layers of the shell are ridged up around the posterior angles at 

 the ends of the hinge line of the embryonic shell, showing that the agency of the mantle 

 was exercised in adding to the margins. In this stage of growth the shell was still quite 

 thin, and the several stages of growth are clearly marked by the imi^ression of the gradu- 

 ally enlarging previsceral depression, now less distinctly indented than in the embryonic 

 shell ; we can trace the growth of the shell by the anterior and posterior points of the 

 pre-visceral depression, as well as by the two series of scars, diverging from the posterior 

 margin that mark the periodical change in the position of the adjustor muscles (figs. 1/ 

 and g.) 



Owing to the thinness of the shell at this early period of growth, the frequent 

 enlargements of the shell are shown by the defined margin of each shell layer ; of these 

 about six can be distinguished on the inner half of this zone of the shell. The surface of 

 the shell in this part bears the rasp-like ornamentation characteristic of the species, on all 

 except the last one or two lajers, where the growth is indicated only by ridges concentric 

 to the umbo, and here there is no radirlated surface. The rasp-like ornament on these 

 shells is not unlike that on Acrothele, to which this species is related ; but iu Acrothele 



Sec. IV, 1889. 20. 



