Jackson.] 536 [April 4, 



ductor of the adult. Unio also seems to be peculiar in having its 

 shell originate as an undivided membranous cuticula, 1 whereas the 

 Lamellibranchiate shell is typically divided from its initial stages 

 onward (see Horst (6 and 7),Lankester (17), Hatschek(5) and other 

 authors). It is anomalous in having the primitive oral invagina- 

 tion on the dorsal aspect of the embryo and in having the byssus 

 gland originate in the posterior end of the body. So many pecu- 

 liarities exist in the embryo of Unio, it may, we think, in view of 

 the evidence, be safety assumed that the development of the poste- 

 rior muscle first, is not typical and, therefore, not incompatible 

 with the theory that the anterior is typically first developed in La- 

 mellibranchs. 



In Ostrea the mouth and anus develop ventrally, the anal ex- 

 tremity of the intestine revolves dorsally, the anterior adductor 

 develops, at a stage not yet determined the velum disappears. The 

 mouth and oesophagus revolve dorsally and stop on the ventral 

 side of the anterior adductor. The posterior adductor has devel- 

 oped apparently on the ventral side of the intestine. When one 

 compares this with the figures and descriptions of the development 

 of the above mentioned genera (excepting Mytilus and Unio), to- 

 gether with the anatomy of the adults, it seems that there is a 

 close uniformity of plan. The genera mentioned are not numerous, 

 but they have been taken from widely separate groups of Lamelli- 

 branchs. There is then strong evidence in favor of the development 

 of the anterior adductor first in Lameliibranchs, as the typical 

 mode, and the development of the posterior adductor later, after the 

 intestine has revolved into place, from its early ventral position. 

 This theory also easily explains the constant relative positions of 

 mouth and anus to the two adductors in the adults of dimyarians, 

 or to the single adductor, where only one exists, as in monomya- 

 rians. This first formed muscle being the morphological equivalent 

 of the anterior muscle of adults which retain it, and the evidence for 

 its typical uniform first appearance being so strong, since it is 

 presumably of phylogenetic importance, I venture to suggest for 

 this early stage the name antemonomy avian. 



It is seen that I have so far described two distinct stages of de- 

 velopment in the anatomy of the young oyster. First, a single- 

 muscled stage, the antemonomyarian, fig. 1, pi. iv, pointing towards 

 a problematical ancestor, which in the adult condition had only 



iThis same feature lias been observed by Professor Brooks in Anodonta (2). 



