2H2 CEYLON PEARL OYSTER REPORT. 



structure is different and it is monoecious whereas the Lucinidae are dioecious, and 

 there are other anatomical characters in which it differs from Montacuta (see 

 PELSENEEB, 11, pp. 203, 204). 



The only other members of the Submytilacea in which the external demibranch is 

 wanting are the Corbidae and Scioberetia. Jousseaumia has clearly no affinities with 

 the Corbidae, but, as has been pointed out, it has certain features in common with 

 Scioberetia. Both are hermaphrodite, commensal or semiparasitic, have a similar gill 

 structure and a single pallial suture, but in Jousseaumia the mantle is not reflected 

 over the shell and it therefore must be excluded from the Galeommidse, to which 

 Scioberetia belongs. 



The balance of evidence is in favour of placing Jousseaumia among the Erycinidae 

 in spite of the absence of the external demibranch. This last character, taken by 

 itself, is of no systematic importance, since it occurs in forms as far apart as Ludna, 

 Scioberetia, and Teredo. BJDEWOOD has shown that the external demibranch is liable 

 to modification and partial suppression in a large number of widely separated genera, 

 and its total suppression may well be accounted for by changed conditions of life 

 affecting the respiratory and alimentary functions. I have shown that there is 

 evidence that the gill is degenerating in Jousseaumia, and that the reflected lamella 

 of the existing demibranch, never very well developed, is rudimentary in a certain 

 number of adult individuals. The conditions which are causing the degeneration of 

 the reflected lamella of the inner demibranch may well have caused the total 

 suppression of the outer demibranch. On the other hand, the details of the gill 

 structure agree very closely with those of the Erycinidae, particularly with that of 

 Lasffa, and the internal ligament, the shell characters, the hermaphroditism and 

 other anatomical features point to a close relationship, particularly to the last-named 

 genus, in which the external demibranch is very short and has no reflected lamella. 

 It may be further observed that Jousseaumia presents an interesting example of the 

 admixture of primitive and specialised characters which is so puzzling to the 

 systematist. RIDEWOOD rightly regards the gills of Astarte as being among the most 

 primitive of all Eulamellibranch gills. In their essential structure the gills of 

 Jousseaumia ^re still more primitive, but at the same time they are specialised, and 

 specialised in the direction of reduction and degeneration, as is shown by the absence 

 of the outer demibranch, the slight development, and even the suppression of the 

 reflected lamella of the inner demibranch, which in some individuals is only repre- 

 sented by a continuous sheet of tissue reflected and attached to the body wall in the 

 region of the foot. It is obvious that this kind of reduction, if carried still further, 

 would lead to the condition found in the Septibranchia, though I do not mean to 

 suggest that Jousseaumia is closely related to this order. 



As other evidences of primitive characters we may note, in Jousseaumia, the relics 

 of paired resophageal pouches (if I am right in regarding the lateral grooves in the 

 esophagus as such), the obvious cerebral and pleural moieties of the cerebro-pleural 



