406 Mr. Gray on the Structure of the Pholades. 



they are very differently distributed, though in an equally con- 

 tinuous series. The ovulum and the embryo are formed and 

 developed in the sexual canal, and the fetus out of it. The 

 matrix is the third pouch in the normal mammifera, the pouch of 

 incubation and nourishment; the marsupium becomes this third 

 pouch with respect to the second. The difference therefore is 

 solely in the foetal domicile ; we call it matrix in the one, and 

 pouch, or marsupium in the others. 



As to the vestiges of placental organization that I have ob- 

 served, I have seen, and my figure distinctly shews, a certain 

 quantity of papillae. Are these the suckers of a placenta which 

 have not had time to be effaced ? This organ could not have 

 lasted long enough to have grown in the same proportion, and 

 produce a long umbilical cord, as in other mammifera. This 

 placenta would have been sessile, an expression I borrow from 

 the botanists ; it would have been fixed to the lower belly, and 

 in some measure, have grown like certain flowers which live im- 

 planted on woody substances, without an}' intermediate pedicles. 



Or the vestiges that I discovered, might be merely the umbilical 

 cicatrix; and in that case the placenta and its cord would have 

 already perished and disappeared. Only direct observation can 

 establish either of these hypotheses. 



May we not hope that Naturalists, throughout the two 

 Americas, India, and New Holland, will second our efforts to 

 complete the observations we have made, and which can only 

 be made advantageously where Marsupial Animals abound. 



Art. L. Observations on the Structure of the Pholades. By 

 John Edward Gray, Esq. M. G. S. 



There is no family of Conchophorous Mollusca, where the 

 means by which they open and shut their shells have been so 

 much misunderstood, and consequently involved in so much 

 obscurity as the Pholades. This obscurity arises from the dif- 



