1898.] NATURAL SCIENCES OF PHILADELPHIA. 443 



male from an immature female, as is shown in Plate XXIII, where 

 immature individuals are shown in the first row, mature males in 

 the second, aud an immature female at the right end of the third 

 row. To the left of this immature female are shown a number of 

 individuals in which the older part of the shell has the male char- 

 acters, while the newer part has those of the female. " In such ani- 

 mals the penis is usually very small, and, in some cases, has almost 

 entirely disappeared. Quite a complete series of stages in the de- 

 generation of this organ was observed, from the fully-formed organ 

 on the one hand, to a minute papilla on the other. Sections of such 

 animals show that neither male nor female sexual cells are produced 

 at this time. The evidence seems to favor the view that we have, 

 in these cases, an example of proterandric hermaphroditism, but I am 

 not able to assert that this is really the case, although I have spent 

 much time in attempting to decide it." 8 Further, I have not stud- 

 ied a sufficient number of cases to be able to decide whether this is 

 a regularly occurring phenomenon or only an unusual and abnor- 

 mal approach to hermaphroditism. 



Explanation of Plates. 



The plates are from photographs of actual specimens, and are re- 

 duced about one-third in size. 



Plate XXI. Row 1. C. convexa from exterior of Illyonassa. The 

 shells are deeply pigmented and highly arched ; 

 3d to 6th show males attached. 



Row 2. C. convexa (C. glauca Say) from flat sur- 

 faces, some from exterior of oyster shells. The 

 shells are unusually flat and broad, and those 

 from the oyster shells are light in color and mot- 

 tled with brown spots. 



Row 3. First five shells are C. adunca, all with 

 males attached. Remainder of row and all of 



Row 4. C. navicelloides ; many of the shells irregu- 

 lar in shape. 



Row 5. C.fornicata; various sizes, shapes and colors. 



Plate XXII. All shells on this plate are of C. plana. 



Row 1. Interior views of shells of very different 



shapes, due to the characters of the surfaces of 



attachment. 

 Row 2. Exterior views of same. 



8 Embryology of Crepidula, Jour. Morph., Vol. XIII. 



