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 imperialis, Chemn., in the Society's collections, show 

 the remarliahlc condition of being perfectly eqiiivalvnlar. Both valves have ears on 

 the hinge line and thongh 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 equivalvular shell which 

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



The conclusion an'ived 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 i-etains in a 

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

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

 lus ; but from 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. xxvn, 

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

 distinct prodissoconch, similar in form to that of Pecten and Spond} lus, exists. It is 

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

 as shown in the figure, bear a considerable resemblance to yoiing 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 Plicatula, also series of specimens have been traced 

 from the very young to the mature condition. The prodissoconch umbos in specimens of 

 Plicatula, when attached to the object of fixation, PI. xxvii, 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 

 posterioi'ly relatively to the anatomy which is known directly or by analogy to have ex- 

 isted at the close of the prodissoconch period. 



The early nepionic stage of Plicatida, PI. xsvir, fig. 5, is very similar in appearance 

 to a young oyster, as stated. The lower i-ight 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 observeil by authors, that the upper valve served specimen of PUcattda marginata, from the Tertia- 



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



valves, as Crepidula, frequently siuuilates the outlines of exactness the structure of a Bryozoon over which the at- 



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



body, Ix'ing perfectly reproduced in the free valve. The made, but each cell is outlined in the njiper valve of the 



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



