1888.] 539 [Jackson. 



valves and at an angle of about ninety degrees from the hinge axis. 

 This revolution of the axes Ostrea is seen by comparing figs. 1, 2, 

 and 3, pi. iv, and fig. 18, pi. vn. 



In other monomyarians, as Pecten and Anomia, the mouth lies 

 close up under the umbos, which are in the median plane of the 

 shell ; thus the relations of the axis to the shell are similar to those 

 of Ostrea. 



In heteronryarians, as Mytilus, Modiola, fig. 16, pi. vn, Perna, 

 fig. 17, pi. vn, and Avicula, a distinctly transitional series may be 

 studied, in which the relations of the axes are changed from what 

 exists in the typical dimyarians, fig. 15, pi. vn, to what we find 

 in the single-muscled group, fig. 18, pi. vn. 



In the clam {Mya areyiaria), fig. 15, pi. vn, the adductor muscles 

 lie at either end of the longer axis of the shell, the mouth lies close 

 up behind the anterior adductor, the anus overlies the posterior 

 muscle, the umbos lie dorsally. The antero-posterior axis is nearly 

 parallel to the hinge axis. 



It is seen that in Modiola plicatula, fig. 16, pi. vn, the mouth and 

 anterior adductor have revolved dorsally, so that they lie much nearer 

 to the umbos than in the clam, fig. 15, pi. vn ; similarly, the poste- 

 rior adductor and anus have revolved in the opposite direction, 

 and occupy a position much further removed from the hinge line 

 than in the clam. The antero-posterior axis lies at an angle of 

 about 25° from the hinge axis. 



In Perna ephippium, fig. 17, pi. vn, the mouth lies close up un- 

 der the umbos, but the umbos are not in the median plane of the 

 shell. No anterior adductor was found, but it probably exists in 

 closely related species. 1 The posterior adductor and anus are still 

 further removed from the hinge line than was the case with Modiola. 

 The antero-posterior axis lies at an angle of about 50° from the 

 hinge axis. 



A revolution of the axes similar to that traced in 0} r sters, and 

 in the above serial groups, may be seen in Mulleria lobata, a widely 

 separate genus. According to Adams (1) and other authors, it 

 has two muscles when young ; but, when fully grown, one muscle. 



1 Mr. Pnrdie, of New Zealand, in a paper (19) recently published, notes that Mytilus 

 latus has no anterior adductor, while two otherwise closely related species M. edulis 

 and M. magellanicus, have anterior adductors. In M. latus, the loss of the anterior 

 muscle is accompanied by a movement of the one existing muscle inwards, nearer the 

 central plane of the valves, where its mechanical action is more effectual. 



