388 EGBERT TRACY JACKSON ON THE 



tainecl by following the lines of growth) had a long hinge line, with a comparatively slight 

 development of the ears. 



Crenipeeten, Hall, is a genns like Avicnlopecten in form, but with crenulations on the 

 hinge mai'gins. Pernopecten, Winehell, has a large triangular cartilage pit and a crenu- 

 lated liinge plate. The crenulations in these genara are described as cartilage pits. A 

 crenulated hinge plate is found in the young of Pecten tenuicostatas, fig. 38, p. 348, and 

 P. monotimeris. It has also been found more or less developed in the young of several 

 recent species of Pecten by Dall (16). He considers the crenulations as an order of denti- 

 tion' and the crenulations in the fossil genera cited are probably of similar significance. 

 As evidence against the view of considering them as cartilage pits, it should be observed 

 that in Perna, etc., where numerous cartilage pits are typical, the primitive pit as well as 

 succeeding ones assumes the pei'pendicular sides characteristic of the pits of the genus, 

 figs. 30-31, p. 329, and PI. xxvi, fig. 16. Crenipeeten and Pernopecten are considered as 

 side issiies from Avicnlopecten. 



Lima has many features of anatomy and shell form linking it with Pecten; it is, there- 

 fore, presumably a side issue from the early members of this genus. (Further descrip- 

 tion of Lima, see p. 353.) 



The genus Chlamys (Bolton), characterized by the great development of the ventral 

 (anterior) ear and correlated I'eduction of the dorsal ear, is an undoubted branch from 

 Pecten. When young, PI. xxvni, fig. 14, both ears are nearly equally developed as in 

 Pecten. Chlamys is a swing in one direction as Lyriopecten is a swing in the other di- 

 rection, for the latter has the anterior ear very reduced and the posterior ear largely de- 

 A^eloped. (Further desci-i]ition of Chlamys, see p. 348.) 



Yola (Klein) is a near ally of Pecten ; in the adult it is highly incquivalvular, the right 

 valve is deejDly concave, while the left valve is flat or even slightly concave. AVhen young, 

 however, it is nearly or quite equivalvnlar. In a finely preserved specimen from Japan, 

 a pi'odissoeonch and nepionic stages were observed in both valves, similar to those char- 

 acteristic of Pecten irrad'ians, PL xxviii, figs. 9-12. 



Amusium (Schumacher) (see Dall) is characterized by radiating, internal ribs, a short 

 hinge line, nearly equal ears and a high reduction or complete absence of the byssal notch. 

 The ears of the right valve may be produced beyond the limits of those of the left, on the 

 hinge border, as in Amiisiiun japonicnm, thus making an abbreviated approach to the 

 condition charactei'istic of the genus Entolium, Meek. Commonly a pair of ridges di- 

 verge from the hinge line and terminate in tubercular processes close to the auricular 

 sinus of either eai-. Dall has named them auricular crurje. Crura; similar to those of 

 Amusium may be traced in most species of Pecten, though they are often very reduced, 

 the terminal tubercle at the auricular sinus alone existing. The function of the crurae 

 is not understood. Dr. Dall, in his studies of deep-sea Amusium, notes the existence 

 of prismatic structm'e in several species, in which, however, it was not limited to the I'ight 

 valves, as shown in our studies of young Pecten (section x). Amusium is considered 

 a side issue from Pecten, rather than in the line of ascent of the group. 



Anomia is doubtless a side issue from Pecten as discussed (section xin). In the 

 young there is, as in Pecten, a deep byssal sinus in the right valve, PI. xxix, fig. G. 



' St'C ivCcroiiCL- to ii ijtipir by \V. U. iJiiU, foot-uotL', p. 37G. 



