1888.1 535 [Jackson. 



myarians. As noted above, the anal part of the intestine revolved 

 dorsally during the early development of the oyster, and it is evi- 

 dent that the posterior muscle when formed must have originated 

 on the ventral aspect of that organ. If the posterior muscle were 

 developed on the dorsal side of the intestine it would always have 

 retained that relative position, as the permanent anterior adductor 

 of dimyarians does in relation to the mouth and oesophagus. It 

 does not seem likely that the intestine would first force itself past 

 an existing muscle, and then, when it got on the dorsal side, wrap 

 its terminal part around the dorsal aspect of that muscle, as it does 

 in all adult Lamellibranchs. One of the striking characteristics 

 of the ostrean prodissoconch is, that the umbos point posteriorly, 

 as shown in fig. 2, pi. iv. This is contrary to all figures of devel- 

 oping dimyarians that I have seen, as in them the umbos point an- 

 teriorly. 



In Ostrea indisputably the anterior adductor is first developed. 

 In Cardium after Loven (18), as mentioned by Huxley (8), also 

 in the figures by Loven of Mocliolaria and Montacuta, in Pisidium 

 after Lankester (17) and in Anodonta according to Schierholz 

 (26), the anterior adductor is first developed. I have only found 

 two exceptions to the general rule of the anterior being developed 

 first. Lacaze-Duthiers, in his paper " Sur le Developpement des 

 Branchies" (15), figures a young Mytilus edulis with a posterior 

 adductor, but no anterior. He notes, page 21, that he did not 

 observe any anterior, and believes that the posterior adductor is 

 first developed. He explains it, by the theory of arrested devel- 

 opment, and compares this stage with the monomyarian Lamelli- 

 branchs, Ostrea and Spondylus. In Loven's figure of Modiolaria, 

 the anterior adductor is first developed, and, as it is so closely 

 related to Mytilus, it seems possible that the distinguished author- 

 ity, Lacaze Duthiers, overlooked the anterior adductor of Myti- 

 lus. Again, his comparison of the single-muscled stage of Mytilus 

 with Ostrea, as a case of arrested development, will hardly hold, 

 as Ostrea in the young is typically dimyarian, and therefore must 

 be considered as a branch from a dimyarian root, not an ancestral 

 monomyarian type. At any rate, if I am not mistaken, Mytilus 

 must be considered as an exception to the general rule. The second 

 exception I have found is Unio, where Huxley (9) in his " Anatomy 

 of Invertebrated Animals," page 416, quoting Rabl (20) sa} T s, that 

 the adductor muscle is first single and answers to the posterior ad- 



