G. A. DREW ON YOLDIA LIMATULA, SAY. 29 



"2. Foot has a ventral 'creeping' disc (not existing in others 

 except Pectunculus)" 



"5. Absence of a posterior aorta." 



"7. Genital glands opening into the pericardial extremities of the 

 kidneys." 



" 8. Pleural ganglia still distinct from the cerebral, and pleuro- 

 pedal connectives arising by distinct origins from the cerebro-pedal." 



Again on the same and succeeding pages : 



"2. With the Nuculidte (Leda, Yoldia, etc.), one sees again that the 

 retractors of the foot (columellaires) form an almost continuous series 

 (without specialization) between the two adductors." 



"6. With the Nuculidas the otocysts open freely to the outside." 



I can do no better than to refer the reader to what has already 

 been said on these subjects (see pages 8, 16, 18 19, and 21). The foot 

 muscles of Yoldia are surely among the most highly developed of any 

 to be found in the Lamellibranchia, and the only reason they occupy 

 so much of the dorsal margin is because of their enormous size. 



EMBRYOLOGY. 



The most striking peculiarities in the development of Yoldia are 

 connected with the formation and disappearance of the test. 



I find similar tests in the embryos of two species of Nucula (N. 

 proxima and N. delphinodonta), in both of which cases the tests are 

 cast. In the former, the eggs are thrown free in the water, and the 

 embryos closely resemble those of Yoldia, but are considerably smaller. 

 In the latter species, the embryos are much larger, and are somewhat 

 modified by being carried in a mucous case, which is attached to the 

 posterior end of the shell. 



Outside of the group, so far as I have been able to learn, Dondersia 

 is the only other Mollusk whose embryo is known to be provided with 

 a similar test. 



Comparing the embryo of Yoldia with PruvotV 2 account of the 

 embryo of Dondersia, we find that the fully formed tests of both are 

 composed of five rows of cells, all of which bear cilia. Counting from 

 the anterior, the cilia on the third row of cells in the test of Dondersia, 

 and the second, third and fourth rows of cells in the test of Yoldia, 

 are long and collected into bands which surround the embryos. The 

 body of the embryo of Dondersia protrudes posteriorly during develop- 

 ment. No such protrusion takes place with Yoldia. Each is provided 



