PHYLOGENY OF THE PELECYPODA. 309 



fig'. 12, where it iiniiediatcly underlies the umbos, and opens toward them is merely a 

 series of mechanical stages. Correlatively with the dorsal revolution of the mouth the 

 anterior adductor muscle has disappeared in the growing oyster. 



In the dimyarian stage of the young oyster, PI. xxiv, figs. 1-2, the relative position of 

 the axis of the body to the hinge axis of the shelP resembles that of adult t3^pical dim- 

 yarian Peleeypoda,'- PI. xxv, fig. 9, where the hinge is dorsal, free edges of the valves 

 ventral, the mouth and anus l^'ing at either end of the equal valves, so that the antero- 

 jiosterior axis' is nearly parallel to the hinge axis. In the adult oyster, PL xxv, fig. 12, 

 this relation is difierent. The mouth lies close up under the hinge line, marking this as 

 the anterior end of the shell. The free ends of the valves are at the posterior extrem- 

 ity, the gills lie on the ventral side, and the intestine on the dorsal.* The antero-pos- 

 terior axis passes nearly through the umbos and centre of the valves and at an angle of 

 al)0ut ninety degrees from the hinge axis. 



To bring about the adult condition the antero-posterior axis has therefore revolved 

 through an angle of about 45° from the position it had in the dimyarian stage. In this 

 I'evolution the anterior adductor has disappeared and tlie posterior adductor has become 

 sub-central, thus occupying the fnnctionally most effectual position. In other monomy- 

 arians, as Pecten and Anomia, PI. xxix, figs. 1-2, the mouth closely underlies the umbos, 

 which are in the median plane of the shell ; thus the I'elations of the antero-posterior 

 axis to the shell are similar to those of Ostrea. 



In heteromyarians, asMytilus, Modiola, PI. xxv, fig. 10, Perna, PI. xxv, fig. 11, and Avi- 

 cula, a ti-ansitional series may be studied in which the relations of the axes are changed 

 from what exists in the typical dimyarians, such as Mya, PI. xxv, fig. 9, to what we find 

 in the single-muscled group, as Ostrea, PI. xxv, fig. 12. 



In the clam, Mtja areuaria, fig. 9, the adductor muscles are at either end of the lon- 

 ger axis of the shell, the mouth lies close behind the anterior adductor, the anus over- 

 lies the posterioi- muscle and the umbos lie dorsally. The antero-posterior axis is nearly 

 parallel to the hinge axis. It is seen that in Modiola, PI. xxv, fig. 10, the mouth and an- 

 terior adductor have revolved dorsally, so that they lie nearer to the umbos than in the 

 clam. Similarly the posterior adductor and anus have revolved in the opposite direction, 

 and occupy a position much farther 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 ephip- 

 pium, PL xxv, fig. 11, the mouth lies close up under the umbos, but the umbos are not 

 in the median plane of the shell. No anterior adductor was found, but it pi'obably ex- 

 ists in closely related species.^ The posterior adductor and anus are still further re- 



'Tlie hinge axis refers to au ideal line drawn throngh rior end of the body has already been pointed out by sev- 



tlie hinge area, and coinciding with the axis of motion of eral authors, and Hyatt (28) figures au oyster drawn over 



the valves, as shown in PI. xxv, figs. 9-12, inclusive. a clam to show their relations. Ryder (63), on page 787, 



-I say resembles because the mouth does not closely un- notes the rotation of nearly ninety degrees that must take 



derlio the anterior adductor muscle, as the velum inter- place from my flg. 25, p. 300, after Huxley, to bring about 



vcnes, and therefore the comparison is not exact, though the adult relation of parts. 



to all intents and purposes the comparison is perfectly °Mr. Purdie, of New Zealand, notes that J/(/(!7i(S Znt«s has 



correct. no anterior adductor, while two otherwise closely related 



^The ant'.'ro-posterior axis, as shown in PI. x.xv. figs. species il/. eiZjiZis, and Jf. ii!a(7eH«(i!c?is, have anterior adduc- 



9-12, is considered as passing throngli the moutli and mid- tors. In M. lotus the loss of the anterior muscle is ac- 



dle of the ])ostorior adductor nniscle and nearly or (jnite conipanied by a movement of the one existing muscle 



coinciding witli the termination of the intestine. in\v;inls, nearer the central plane of the valves, whei'e its 



••'I'lie fact tlial the nnil)os in tlie oyster are at the ante- nieelianical action is more ett'ectual. 



MEMOIIIS UOSTON SOC. NAT. UIST., VOL IV. 42 



