Jackson.] 532 [Apr. 4, 



The stages described and named in that essay, as the protem- 

 bryo, mesembryo, metembryo, neoembryo and typembryo, are to 

 be found in following the early development of the oyster, though 

 the concentration of development 1 is so great that they overlap one 

 another. In the present paper the first application of the theory 

 will be to the early single-muscled stage. 



The first oyster material which I had for study was some very 

 young spat sent to Professor Hyatt, by Prof. J. A. Ryder. During 

 the summer of 1887, a large amount of material was collected at 

 Buzzards Bay, Massachusetts, where oysters were secured on ar- 

 tificial cultch of pottery, wood and glass, 2 as well as on natural 

 cultches of shells and stones. The studies of fossils were made while 

 arranging collections as Professor Hyatt's assistant in the palse- 

 ontological department of the Museum of Comparative Zoology. 

 I also had free access to the collections of the Boston Society of 

 Natural History. 



Before turning to scientific considerations, a point in the life-his- 

 tory of the oyster is worth noting. One of the most successful 

 spatting grounds at Buzzards Bay, is a sand spit in South Ware- 

 ham, formed by the dividing waters of the tide, as it rises and flows 

 on the one side to Buttermilk Bay, and on the other to Onset. The 

 strength of the tide is very great at this point. The bar at low tide 

 was bare for about four hours during the time when oysters were 

 setting most abundantly late in July and early August. Here, then, 

 the young oysters, which are exceedingly small when first attached, 

 bear exposure for several hours daily to the air and sun, which dries 

 up all appearance of moisture on the rocks and shells to which they 

 are attached. Further, the spat not only sets and grows but de- 

 velops quite as rapidly during summer as in localities covered by 

 water all the time. They are removed in late autumn by the oys- 

 termen to deeper water, and no oysters reach maturity on the bar. 



Lamellibranchs are divided into three groups, according as the 

 adults have a single adductor muscle, monomyarians ; two equally 

 developed muscles, dimyarians ; or again, two muscles, one large 

 and functionally the most active, the other smaller, heteromyarians. 3 

 It is of great interest to know what are the relations between these 



*For Professor Hyatt's theory of the concentration and acceleration of development, 

 see reference No. 12. 



2 The details of the methods of procedure have been briefly described in "Science" 

 since this paper was read. See reference No. 14. 



3 Those grouped as heteromyarians do not always have two muscles. 



