Journal of the Philadelphia Academy/, ^Q5 



surfaces. The form of the float varies in the species respectively. 

 In the J. fragilis it is convex, subcarinate above, and concave 

 beneath, straight, and composed of large vesicles ; in /. globosa^ 

 the vesicles are smaller, the float is flat above and beneath, and 

 by the re-union of one of the edges, it is formed into a spiral and 

 nearly circular disk : and in J, exigua^ it is straight, narrow, and 

 flattened, and the vesicles are small. Along the under surface of 

 the float, a little line of pearly fibres is remarised, to which are 

 attached the eggs of the animal. Dr. Coates has had no oppor- 

 tunity of observing those of the J. fragilis, but he is strongly 

 inclined to believe that the eggs figured by Sir E. Home in the 

 Philosophical Transactions for 1817, belong to some other marine 

 animal. He grounds this belief on the dissimilarity between these 

 figures, and the eggs of the J. globosa and the J. exigua. In the 

 two last-mentioned species the eggs are contained in little mem- 

 branous bags of some consistence, which are attached in rows to 

 the pearly fibres of the under surface of the float by small filamen- 

 tous peduncles of an appearance similar to that of the fibres. 

 These bags are covered with minute gelatinous, conical eminences, 

 and are partially divided by incomplete septa, as may be dis- 

 covered by means of a powerful lens. In the J. exigua the 

 division is very partial; but in the J.- globosa it gives the whole 

 sac a chambered appearance. It would seem that the animal 

 occupied considerable time in the deposition of its eggs, the bags 

 nearest to the extremity of the float being constantly found 

 empty, while the central ones contain young shells fully formed, 

 and those towards the animal are filled with the eggs. It appears 

 probable that the young animals, when liberated from their 

 chambers, ascend the float of the mother, and in this way gain 

 access to the surface, and construct the elements of their future 

 support. 



The other paper is from the pen of Mr. Say. It is *^ On a new 

 species of Modiola," and describes, under the name of iVf. opifex^ 

 a shell nearly allied to the M. discors and M. discrepans. It 

 is a native of Minorca, and was found inclosed in a conical mass 

 of fine agglutinated sand attached by a broad base to the surface 

 of a Peden. 



