WOODWARD : EPHIPPODONTA. 21 



The Mantle mid muscular system. — Yentrally the mantle folds are 

 distinct from one another for about ttiree-quarters of their length, in 

 the posterior fourth they are fused together (Fig. 1, vi.") but separate 

 again at the extreme posterior end of the body. In this way two 

 openings are left between the folds, a small posterior one corresponding 

 to the exhalent siphonal (anal) aperture (Figs. 2, 5, 6, 16, sp.) and a 

 large antero-ventral one representing the pedal, byssal and branchial 

 openings (branchio-pedal aperture). 



The anal aperture appears at first sight to be devoid of a siphon, 

 but microscopic examination of transverse sections (Fig. 16, sp.) shows 

 that there is in reality a small rudimentary one present, so small that 

 it is not in all probability protrusible beyond the mantle edge. Its 

 presence is, however, of interest, as we find a single but small siphon 

 present in Galeomma. 



The margins of the mantle are enormously thickened and muscular 

 and the extreme edge is beset with sensory papillae, the inner border 

 is produced inwardly as a strong muscular shelf, which is probably 

 capable of great contraction and with the foot closes the gaping 

 aperture between the valves in the contracted condition (Figs. 8 to 10). 



The pallial muscle is well developed, and presents an imbroken line 

 extending in an arc from the anterior to the posterior adductor muscle. 

 The adductor muscles (Fig. 5, pa. ad.) are small and incapable of 

 closing the valves ; the anterior is slightly the largest ; in both a 

 dorsal portion is slightly differentiated from the main mass of the 

 muscle. 



The pedal retractors (Figs. 2 and 5, ar. pr.) are well developed, 

 the posterior being the largest. A large protractor pedis {pp.) is 

 also present. 



Perhaps the most striking feature in connection with the mantle is 

 seen in its relation to the shell. Careful microsco]3ic examination of 

 transverse sections of decalcified specimens reveals the fact that a thin 

 layer of the mantle is reflected over the outer sides of the valves, 

 and completely covering them comes into contact and fuses with the 

 body-wall between the two valves in the mid-dorsal line (Figs. 8 to 

 16, m'). Thus each valve is completely enclosed in a fold of mantle, 

 or in other words the shell has .become an internal one. 



The fold of the mantle covering the shell consists of a double layer 

 of cells with a thin layer of connective tissue between them. The 

 outer layer is further produced into numerous club-shaped papillae, 

 which are in all probability sensory. In some few places the calcified 

 spines appear to pierce and project through this thin covering (Fig. 

 10). This may very possibly be due to the somewhat rough handling 

 which the specimen has undergone, and, in some cases, to the explosive 

 nature of the decalcifying process, for I find no definite region where 

 the shell is uncovered. 



The internal character of the shell is a very interesting feature, 

 since there is only one other Lamcllibranch hitherto recorded in 

 which this is the case, viz. : — Chlamydoconcha, Dall.^ Dr. Dall states 



1 Science, iv. 1884, p. 50-51. 



