A. E. Verrill — Study of the family Pectinidce. 63 



auricles, which are small and nearly equal, and apparently differ but 

 little in sculpture, but the actual sculpture of the type is uncertain. 



Many mesozoic fossil species appear to belong to this group. The 

 type is from the North American cretaceous formation. 



Some of the broad fossil species that have been referred to this 

 group appear to belong rather to Protamiishmi. These have line 

 concentric grooves and lamellre, and equal auricles, but no radial 

 sculpture. 



Several living delicate deep-sea species are perhaps nearly allied to 

 this groujj. 



As the real structure of the type of the genus is still uncertain, I 

 have thought best to make a new genus for the recent forms, under 

 the name of HycdopeoMn (see p. 71). 



Among the known species are H. undatus V. (pi. xviii, fig. 5), 

 H. dilectus V. and B., H. fragilis (Jeff.), H. pudicus (Smith). 



The group of shells here indicated forms a very definite division, 

 worthy, perhaps, of generic rank. These recent species, do not show 

 the camptonectes-sculpture, but both valves are distinctly undulated. 

 Some of these species are otherwise smooth, but others are finely 

 radially striated. They are all hyaline, very thin, and very simple 

 in structure. 



Pernopecten Winchell, 1865. Type, P. limiformis W. 



Pernopeden Winch., Proc. Acad. Nat. Sci., Philad., for 1S65, p. 125 ; Hall, Pal. 

 New York, v, pt. I, sec. II, In trod., p. Ivii, figs. 1,2. 



The type of this genus has the form of a small round Aimcsmm, 

 or Propeamusium, to which it appears to be closely allied. The 

 auricles are small, short, subequal, poorly differentiated, much as in 

 Amusium. Byssal notch slight. There is a well-developed median 

 resilial-pit. The cardinal ribs bear a series of minute transverse 

 grooves, or " crenulations " on each end, like those of Euchondria, 

 etc. There is a well-developed, rib-like auricular crura on each end. 

 The shell is smooth, or has fine radial stride. 



Burlington Sandstone of Iowa. 



This genus should probably be regarded as the ancestral form of 

 Amtcsiuni and Propeanmsiuni. It was probably a free-swimming 

 species, like the modern genera. 



Lyropecten Conrad, 1867. Type, P. /lodosus (Linne). 

 Liropecten of several later authors. 



Shell lai-ge and strong, corrugated, with large, fluted, and usually 

 nodose, primai'y i-adial ribs, which do not increase in number, and 



