354 



ROBERT TRACY JACKSON ON THE 



has well-developed umbos. The succeeding dissocoiich is limaform, but no prismatic 

 structure was observed as in young Pectens; it may, however, exist in well-preserved 

 specimens. 



Fig. 41. 



Fig. 42. 



Xn. Anomia. Anatomy and Habits. 



Anomite freely attached themselves to the glass slides in my drain-pipe traps and 

 therefore I had an opportunity to study the young under this favorable condition with 

 transmitted light. The species is Anomia glabra, Verrill, figs. 41-42, which is abun- 

 dant in the shoal waters of Buzzards Bay, often covering shells and stones as thickly as 

 do oysters. 



Anomife have not in the same degree as oysters the power of resisting drying which 

 is so characteristic of that mollusc, and rarely set at points which are long exposed at 



low tide. They grow rapidly and some speci- 

 mens attached to glass slides measvn-e 25 mm. in 

 height, the result of tlie summer's growth. These 

 almost equal the adult in size as the latter rarely 

 exceed 40 mm. Examining Anomia^ where a num- 

 ber are attached to a single rock or shell, it will 

 be found that the longer axes of the specimens in 

 most cases are nearly parallel, the leverse of the 

 condition of oyster spats, where the axes point 

 at every angle in the jjlane of the surface of at- 

 tachment. This parallel arrangement in Anomia 

 seems to bear a relation to the direction of strong- 

 est currents, the animals arranging themselves 

 during their early locomotive stage in the plane 

 of least resistance and afterwards becoming permanently fixed in this position. On the 

 other hand, oysters Avhich have no crawling locomotive stage, pay no attention to prevail- 

 ing currents in their position of fixation. A marked case of animals arranging them- 

 selves with regard to currents was observed in Crepidula fornicata, PI. xxiii, fig. 22. 

 Large numbers of colonies were found at Buzzards Bay like the figure in which all the 

 individuals were headed one way. This arrangement in one plane does not alwaj's hold 

 in Crepidnlas as they may be arranged at different angles; in such cases Ave may rea- 

 sonably suppose that no strong prevailing cui-i-ent existed in the location where the ani- 

 mals grew. 



Anomia was foi*merly considered as related to Brachiopods, but such a connection has 

 long been disproven. Forbes and Hanley and Lacaze-Duthiers comijared Anomia to 

 Pecten; but notwithstanding this, a close relationship of the genus to Ostrea has been 

 as a probability quite generally entertained. This assumption appears founded on a 

 certain similarity of the shell of Anomia to an oyster, especially a young oyster. Anomia 

 belongs to the same broad group as the oysters, as proven by features of anatomical 

 and shell structure; but it is more nearly related to Pecten than to Osti-ea. Previous 

 studies have shown that the form of the shell in Pelccypods is often highly modified by 



Fig. 41. Adult Anomia glabra viowt-cl from the 

 right, lower side ; uatural size. 



Fig. 42. The same iu profile view. (From Bin- 

 ney's Gould.) 



