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. 
5. 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 6.5mm., 
the breadth 6mm.; the height of the shellis 3.9mm. The surface is faintly 
wrinkled transversely. True concentric and radiating strice are not shown in 
the specimen. The absence of radiating and concentric strie 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 
strie, passing longitudinally along the shell. In the casts, the transverse 
ridges are less distinct, and the longitudinal striez are not seen at all. When 
not crushed, the apex is broad oval in outline. Diameter of the aperture 
2.5mm.; 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 slightly curved. Shell corru- 
gated into four or five rounded ridges, passing transversely around the shell, 
very strong below, decreasing rapidly in size towards the apex. Greatest di- 
ameter of the aperture, 4mm.; height of the shell, the same. Longitudinal 
striz very fine and closely set. 
Sd 
