836 mocKEiuxas OF THE NATIONAL MUSEUM. 



HYALOPECTEN DJLECTUS Verrill and Bush. 



(Plate XCVII, fig. 9.) 



Hyalopecten dilectus YEKRILL and BUSH, iu VERKILL, Trans. Conn. Acad., X, pp. 

 80, 92, 1897. 



Shell small, thin, fragile, strongly imdulated, slightly oblique, with 

 the ventral margin broadly rounded, dorsal margin straight. In the 

 right valve the anterior auricle is rather narrow, elongated, with a 

 deep angular notch beneath; the posterior auricle is shorter, with a 

 prominent dorsal angle which is less than a right angle, owing to the 

 emargination of the posterior end. In the left valve the anterior auri- 

 cle is broad, with its posterior end nearly rectilinear, and forms a right 

 angle with the dorsal margin; the posterior auricle has a slightly 

 prominent dorsal angle and posterior emargination as in the right valve. 

 The beaks are a little prominent and project somewhat above the dor- 

 sal margin, more in the upper or left valve. The surface in both valves 

 is covered with broad and rather regular undulations, which are most 

 prominent in the left valve, and are crossed by regular, well-spaced, 

 thin, raised, radial lines, which become fine and more crowded at the 

 ends of the valve; they are nearly obsolete in the right valve, being 

 represented by microscopic striae In both valves the anterior auricle 

 is marked by several fine, rough, radial ridges which are stronger and 

 more numerous iu the left valve. The interior is strongly undulated, 

 and the left valve is marked by distinct, radial grooves, liesilium 

 small, central. Color yellow, or dirty white. 



Length, 8 mm. ; height, the same. 



One live, imperfect specimen (No. 52539), station 2570, off Marthas 

 Vineyard, in 1,813 fathoms, 1885. 



This species is closely allied to Hyalopecten fraailis (Jeffreys) and 

 resembles very nearly his figure, 1 which probably represents a species 

 distinct from the original type described by him, and may be identical 

 with our shell. The latter differs decidedly from the original descrip- 

 tion of H. fragilis. Moreover, we have obtained from several stations 

 a shell of similar size which appears to be the true fragilin,- as it agrees 

 closely with the description. Hyalopecten pudicus (Smith) from east of 

 Marion Island, in 1,375 fathoms, is a closely related species, as is also 

 Hyalopecten -undatus Verrill. 



These four species agree in having the valves thin and translucent 

 and sculptured with distinct concentric undulations, while the radial 

 sculpture does not form strong ribs. They seem to be related to the 

 genus or subgenus Syncyclonema Meek, which was based on a creta- 

 ceous species and has not hitherto been reported as still living. But 

 the exact characters of the typical fossil species are not yet known. 



1 Proc. Zoo]. Soc., London, pi. XLV, fig. 1, June, 1879. 



-The true Hyalopecten fragilis (Jeffreys) was taken at five stations between N. lat. 

 40 6', W. long. 68 1' 30", and N. lat. 35 49' 30", W. long. 74 34' 45", in 578 to 1,525 

 fathoms, 1883-1886. 





