828 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.XX. 



HEMIPECTEN Adams and Reeve, 1849. 

 Hemipecien VERRILL, Trans. Conn. Acad., X, pp. 60, 89, 91, 1897. 



Type. Hemipecten forbesianus Adams and lleeve. 



This group includes species with thin, irregular shells attached by 

 the right valve, like Hinnites, but the attachment is effected mainly 

 by a permanent modified byssus. The posterior auricles are nearly 

 obsolete. The byssal notch becomes irregular and nearly inclosed, as 

 in Anomia. 



PSEUDAMUSIUM H. and A. Adams, 1858. 



Pseudammium (pars) H. and A. ADAMS, 1858 (after KLEIN). CHENU, 1862. STO- 

 LICZKA, 1871. ZITTEL, 1881. FISCHER, 1887. DALL, 1886 (pars). VERRILL, 

 Trans. Conn. Acad., X, pp. 60, 90, 92, pi. xvn, figs. 8, 8a, 1897 (restricted). 



Type. Pseudamusium e.roticum (Chemnitz, Lamarck). 



The typical species of this group have nearly smooth, round, sym- 

 metrical, closed shells with well defined, small, straight, obtuse-angled 

 auricles. The valves are nearly equal and have nearly simple, 

 even margins. The external sculpture consists of small radial stria' or 

 riblots, without strong angular ribs and grooves, and it may differ 

 on the two valves. Some of the species show the fine divergent 

 "camptonectes sculpture" on one or both valves, especially when 

 young. The margin is not scalloped, or but faintly so, and there are 

 no definite internal ribs. The hinge-plate has usually but one longi- 

 tudinal fold on each end which is feeble and nearly parallel with the 

 marginal ligamental groove and is usually cross-lined. The byssal 

 notch is small and the pectinidial teeth vary from one up to five in 

 number, or sometimes may be lacking. 



CAMPTONECTES Meek, 1864. 



f'amptonectes MEEK, 1864. STOLICZKA, 1871. ZITTEL, 1881 (type, arenatus Gold- 

 fusB). VERRILL, Trans. Conn. Acad., X, pp. 62, 90, 91, 1897. 



Type. Camptonectes lent (Sowerby). 



Shell subovate, plain, not corrugated, and without strong radial 

 ribs; margin nearly plain. Valves subequal. Auricles unequal; byssal 

 notch well developed. Surface of the shell covered with fine, obliquely 

 divergent, curved, crenulated or vermiculated riblets with intervening, 

 narrow, punctate grooves. 



The curious vermiculated sculpture is not peculiar to this division, 

 but is more or less obvious on the shells of some species of Pseuda- 

 musium. and on species of several other groups, both with and without 

 radial ribs. It is a structural feature that runs obliquely across the 

 ribs and grooves. Most of the species are Mesozoic fossils. 



The recent Pecten striatus and P. tigrimis Lamarck, of Europe, 

 apparently belong to this group, and P. testce might also well be 

 referred to it. The latter is one of the types of Palliolum which 

 might well be regarded as a section of this genus. 



