210 SYNOPTIC COLLECTION. 



Lampsilis radiata with the broad sacs of the outer gills 

 filled with embryos. 



The peculiar characters which separate the larvae of 

 the Unionidae from those of other Pelecypods appear 

 very early in the embryo and are found in both the inter- 

 nal and the external parts. A shell is formed with a 

 straight hinge line. It is provided with hooks which later 

 become a necessity to the larvae. The byssus and spiny 

 beaks form, and peculiar sense organs are developed on 

 the inner surface of the mantle. All this takes place 

 before the animal leaves its parent. This stage is known 

 as the glochidium. Becoming free, the glochidium 

 attaches itself by the byssus, or if fish are near it, fastens 

 itself to the gills, fins, or other parts of the fish by the 

 hooks of its shell and lives the life of a parasite, its host 

 providing the necessary nourishment. All this time the 

 intestine is a closed internal sac and of little use. Later 

 the sac elongates and an opening breaks through. A 

 metamorphosis takes place, two adductors appear in place 

 of one, gills develop, and the peculiar sense organs disap- 

 pear. When fully developed, with the exception of the 

 reproductive organs, it leaves the fish and becomes 

 extremely lively while its further development goes on. 

 The adult shell (No. 406, Lampsilis nasutus Say ; No. 407, 

 L. radiata Gmel.) , has the transverse teeth which prove 

 the origin of the Unionida'e, while the peculiar marginal 

 teeth of the third group are developed later. The hinge 

 of Anodonta (No. 409), however, is so much reduced that 

 it is toothless. 



The third order, in which there are teeth parallel to the 

 margin, is represented by a number of genera. It is 

 important to bear in mind that transverse teeth also 

 usually occur, so that in the differentiated forms the hinge 

 is remarkable for its perfection of mechanism for efficient 

 work. 



Petricola is both a free and a boring mollusc. It is 

 found in soft marshy earth as at Revere Beach and also 



