MUSEUM OF COMPARATIVE ZOOLOGY. 29 



tance from the anterior extremity of the shell. If indeed a Lamellibranch, 

 the specimen is the left valve of the shell. 



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

 one imperfect specimen. 



6. Scenella reticulata, Billings. 



Plate I. Fig. 6. 



Shell small, conical. The aperture is almost circular; but there is a great 

 convexity of curvature on one side, giving rise to a low, indistinct carina, and 

 to a slightly oval outline at the aperture. The apex of the shell is slightly in- 

 curved towards the carinated side. The length of the aperture is 6.5 mm., 

 the breadth 6 mm.; the height of the shell is 3.9 mm. The surface is faintly 

 wrinkled transversely. True concentric and radiating striae are not shown in 

 the specimen. The absence of radiating and concentric striae would at once 

 separate this specimen from the types of the species, but the markings are 

 so delicate that their preservation in this decayed rock would be extremely 

 improbable. In other respects, however, it is very much like the type 

 specimens. 



Locality and position. — Station No. 2, North Attleborough, Mass., Cam- 

 brian, one specimen; also at Topsail Head, Conception Bay, Newfoundland. 



6. Stenotheca rugosa, var. pauper. 



Plate I. Fig. 7. 



Shell small, decreasing rapidly in size towards the apex. Apex strongly 

 incurved. Shell corrugated into from five to eight rounded ridges, passing 

 transversely around the shell. These are crossed by very fine, closely set 

 striae, passing longitudinally along the shell. In the casts, the transverse 

 ridges are less distinct, and the longitudinal striae are not seen at all. When 

 not crushed, the apex is broad oval in outline. Diameter of the aperture 

 2.5 mm.; height, the same. Specimens are often smaller. 



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

 brian, 20 specimens. It is of some interest to note that this form does not 

 occur at Station No. 2, where two other varieties are found. 



7. Stenotheca rugosa, var. abrupta. 



Plate I. Fig. 9. 



Shell small, decreasing rapidly in size towards the apex. The apex never 

 strongly incurved; usually within a moderate distance of a line vertical to 

 the base at its centre. Shell never more than slightlv curved. Shell corru- 

 gated into four or five rounded ridges, passing transversely around the shell, 

 verj' strong below, decreasing rapidly in size towards the apex. Greatest di- 

 ameter of the aperture, 4 mm. ; height of the shell, the same. Longitudinal 

 striae very fine and closely set. 



