MUSEUM OF COMPAEATIVE ZOOLOGY. 31 



of the cast, the outside margin of the coil thickens a little, forming an indis- 

 tinct border along the margin of the shell, which becomes more evident as it 

 approaches the orifice. The edge of the whorl is compressed and rather nar- 

 rowly rounded. The surface of the shell is marked by fine, transverse, closely 

 set striae, which apparently are directed backward towards the earlier formed 

 parts of the shell, but in reality indicate various stages of growth of the shelL 

 The internal cast does not show these fine striae ; but broader and more widely 

 separated elevations, having the same direction as the striae. The diameter of 

 the shell is 3.2 mm. ; the height is a little less than 1 mm. Owing to the shape 

 of the shell it is difficult to measure its height accurately. 



Locality and position. — Station No. 1, North Attleborough, Mass., Cam- 

 brian, one specimen. The discovery of another coiled gasteropod in this divis- 

 ion of the Cambrian of America is of interest, and makes the sudden influx of 

 coiled genera in the Upper Cambrian less inexplicable. 



11- Hyolithes quadricostatus, sp. n. 



Plate II. Fig. 15. 



Shell straight, elongate, tapering gradually to an acute point ; apical angle 

 17°. The external cast of the type specimens is 20 mm. long; including an 

 additional length represented by an internal cast of the same specimen, but 

 extending farther from the apical extremity, it is 25 mm. long. As the end of 

 cast is broken, a length of 30 mm. may be presumed for the entire specimen. 

 The supposed dorsal side is broad and flat or slightly concave along the centre ; 

 when depressed, a low elevation may occur along the median line; in all cases, 

 the surface retains a rather flat appearance. The lateral angles are rounded. 

 The supposed ventral side is as usual flattened along the median line for about 

 half the vridth of the shell. The sides of this flattened surface are more or 

 less elevated, giving it a slightly concave appearance along the median line. 

 Immediately beyond the flattened surface on either side is a more or less dis- 

 tinct groove. The result is, that, in addition to the two lateral angles, there are 

 two angles or ridges on the ventral side, gi^Tng the entire shell the quadri- 

 costate appearance indicated by the specific name. The two ventral ridges in- 

 crease in distinctness as they recede from the apical extremity, and are usually 

 more distinct, or at least less rounded, than the lateral angles. At a distance 

 of six or seven millimetres from the apical extremity, the shell is crossed by 

 an apparently imperforate septum. The cast of this septum from the upper 

 side had the appearance of a flattened surface with a slightly elevated border 

 around the margin. The surface of the shell is marked by fine transverse 

 striae. The longitudinal ridges are less prominent on the interior cast of the 

 shell than on the exterior. 



Taken by itself this species would appear to uc very distinct from the usual 

 forms of Hyolithes, but in reality it forms only the extreme of a series of 

 intermediate species, which begins with specimens characterized by numerous 



