352 ROBERT TRACY JACKSON ON THE 



tortion of the upper free valve is seldom as great as the distortion of the lower cemented 

 valve. Specimens of Spondylus imperialism Chemn., in the Society's collections, show 

 the remarkable condition of being perfectly eqnivalvular. Both valves have ears on 

 the hinge line and though spinous, both are equally so. The right valve has no triangu- 

 lar hinge area or talon and the umbos of the two valves are closely related. Studying 

 these shells, I found no evidence that they had been attached during any period of their 

 growth. With this freedom from fixation we have, therefore, an eqnivalvular shell which 

 is a wide departure from the typical condition of the geiuis. 



The conclusion arrived at that the free valve of attached, cemented Pelecypods is 

 least modified and therefore retains more of the ancestral characteristics, which are nearly 

 or quite eradicated from the cemented valve (section vii) , thus finds strong support in 

 Spondylus. In many figures of fossil Spondylus, published by MM. d'Orbigny and Pictet, 

 (also see PI. xxvi, fig. 1), we find striking dissimilarity in the two valves, the attached 

 one being always the most modified. The free valve which is least modified retains in a 

 marked degree, especially in the young, characters which are traceable to the Pectini- 

 dae, from which the Spondyli have doubtless evolved. I have not dissected a Spondy- 

 lus; but fi'om published notes gather that this genus is closely connected with Pecten by 

 anatomical features*. 



Plicatula is a genus placed next in kinship to Spondylus, the anatomy being similar 

 according to authors. Some very young shells of Plicatula marginata, Say, PI. xxvu, 

 fig. 5, from the Miocene Tertiaries of North Carolina present features of interest. A 

 distinct prodissoconch, similar in form to that of Pecten and Spondylus, exists. It is 

 rounded with slightly developed umbos which are directed posteriorly. The specimens, 

 as shown in the figure, bear a considerable resemblance to young oysters of a similar 

 age; and as it may be thought that a mistake has been made in the identification, I may 

 say that they show the typical teeth of Phcatula, also series of specimens have been traced 

 from the very young to the mature condition. The pi'odissoconch umbos in specimens of 

 Plicatula, when attached to the object of fixation, PL xxvri, fig. 5, point to the right of 

 the observer viewing them from above, whereas in Ostrea, PI. xxv, figs. 1-5, they point 

 to the left, which is due to the fact that Plicatula is attached by the right valve, where- 

 as Ostrea is attached by the left. In both genera, the prodissoconch umbos are directed 

 posteriorly relatively to the anatomy which is known directly or by analogy to have ex- 

 isted at the close of the prodissoconch period. 



The early ne]nonic stage of Plicatula, PI. xxvir, fig. 5, is very similar in appearance 

 to a young oyster, as stated. The lower right valve is flat and closely related to the 

 object of fixation, while the upper left valve is convex.^ No byssal notch is discover- 

 able in the earliest stages of dissoconch growth, and this feature, together with the 

 form of the valves, indicates that Plicatula became permanently attached at the close of 



' It has been observed by authors, that the upper valve served specimen of riicaliila maryinata, from the Tertia- 



of fixed Pelecypods, such as Ostrea and Anomia, also uni- ries, in which the free valve reproduces with beautiful 



valves, as Crepidnla, frequently simulates the outlines of exactness the structure of a Bryozoou over which the at- 



the object of support, ridges, linobs, etc., on the foreign tached valve was growing. Not only a general copy is 



body, being perfectly reproduced in the free valve. The made, but each cell is outlined iu the upper valve of the 



most striking case of this, I have seen, is in a finely pre- I'licatula. 



