PHYLOGENY OF THE PELECYPODA. 389 



Later the byssal sinus becomes enclosed to form the byssal foramen as described (fig. 

 41, p. 354). The peculiarity of the shell and many features of anatomy are clearly at- 

 tributable to the special conditions of environment of the animal (full discussion, sec- 

 tions xii-xni). 



Enigma is an extreme modification in the Anomia line of variation. The right valve 

 is very attenuated, the umbos are far removed from the hinge line, and the byssal fora- 

 men is sub-central. A specimen of ^E ligma cenigmatica, Chemn., in my collection is ob- 

 liquely oval, closely resembling the form found in young Anomia, PI. xxrx, fig. 5. The 

 left (upper) valve from the umbo to the periphery of the shell shows the fusion of the 

 lines of growth that takes place at that area as in young Anomia, PI. xxix, fig. 8. 

 Enigma, therefore, though with full probability a descendant of Anomia, in some char- 

 acters resembles the nepionic period of that genus. 



Placunanomia differs from Anomia in the calcareous union of the borders of the 

 byssal foramen anteriorly; it also differs in the development of diverging lamellae from 

 the hinge border. By the encroachment of the borders of the byssal foramen the byssus 

 in later life tends to become solidly fixed in the lower valve. In Placunanomia macro- 

 chisma, Desh., the byssal foramen is open in specimens two inches in height and is only 

 partially closed in specimens somewhat over four inches in height, as far as observed in 

 the few specimens at command. In the single specimen of Placunanomia rudis, Brod., 

 observed, the byssal foramen is entirely closed. Placunanomia is off the direct line of 

 ascent in the Anomia order of modification, and is transitional between Anomia on one 

 side and Carolia and Placuna on the other, as demonstrated by Fischer. 



Carolia is a genus which seems clearly to be intermediate between Placunanomia and 

 Placuna, as demonstrated by Fischer. When young, Carolia has a deep byssal sinus like 

 adult Anomia. Later the sinus becomes progressively deeper like ^ iiigma, then the bor- 

 ders of the notch become joined on the anterior border like Placunanomia and finally the 

 byssal foramen is cut off by shelly growth, when the shell approaches to the condition of 

 Placuna. 



Placuna in its anatomy is essentially like Anomia but the mantle border has a cur- 

 tain, like Pecten, as shown by Woodward. The structure of the shell is also very simi- 

 lar to Anomia. In the ^ew York State Museum I observed an inclosed byssal plug in 

 young specimens of Placuna sella. The plug occupies the base of a foramen which is 

 filled up by calcareous growth similarly to Anomia; but in the specimens observed the 

 plug was already cut off from the body of the animal by a calcareous secretion that had 

 taken place on the proximal side of that area. A similar condition to that observed in 

 Placuna sella is described and figured by Fischer in young P. i^dpyrO'Cea. The crur^e 

 of Placuna, though existing only in the right valve of the adult (the left valve having 

 corresponding depressions), are equally developed in both valves of the young. The 

 crnrse of Placuna seem to be a modification and posterior extension of the knob-like car- 

 tilage support found in the right valve of Anomia. (Further discussion of Placuna, see 

 p. 362.) 



Dimya, on account of some features of anatomy and shell structure, is placed by 

 Dr. Dall near to Pecten. The shell being osti'caform, he considered that the genus lay be- 

 tween Pecten and Ostrea. I have shown I think conclusively that the ostrean form of 



MEMOIRS BOSTON 80C. NAT. lUST., VOL. IV. bi 



