MUSEUM OF COMPARATIVE ZOOLOGY. 35 
18. Aristozoé ? 
Plate II. Fig. 18. 
What appears to be a hinge line is quite straight, 4.3 mm. long; valve 
oblique, the longest diameter from the anterior end of the hinge line to the 
opposite border is 7.6 mm, and is inclined to the hinge line at an angle of 
about 50°. A large tubercle with broad base and almost pointed extremity is 
situated at a slight distance from the middle of the valve, being closer to the 
posterior extremity of the hinge line than to the opposite parts of the valve. 
Between the tubercle and the hinge line is a depression. <A slight depression 
is found between the tubercle and the posterior border of the shell. A strong 
groove borders the tubercle along its anterior border and extends along its side 
in the direction of the longest diameter of the valve. Half-way between the 
tubercle and that part of the border directly opposite to the posterior extremity 
of the hinge line is a ridge which is well defined towards the border by a 
depression. Two low grooves cross this ridge transversely, connecting with the 
groove at the base of the tubercle. The valve is in a fair state of preservation. 
The affinities of this form are very doubtful. 
Locality and position. — Station No. 2. North Attleborough, Mass., Cam- 
brian, one specimen. 
19. Microdiscus belli-marginatus, sp. n. 
Plate II. Fig. 19. 
Head semicircular, 4.8 mm. long and 5 mm. broad. The glabella is oblong, 
strongly convex, slightly narrowed in front. It is well defined by a deep, dis- 
tinct groove, which continues around the sides and anterior part of the glabella ; 
it is not connected at the front with the groove which lies along the border of 
the head and within the rim. The marginal groove is deep, broad in front, 
gradually growing narrower towards either side; it gives a high relief to the 
rim. The marginal rim has very nearly the same breadth throughout its 
length ; it is beset with small tubercles, usually sixteen or eighteen in number, 
which lie near the interior margin of the ridge. The tubercles directly in front 
of the glabella are often indistinct or obsolete. The occipital furrow behind 
the glabella is low, and not very distinct. It serves chiefly to bring into greater 
prominence a tubercle on the middle of the occipital ring. This tubercle is 
directed backwards, varies in size, and is often low, and again may become a 
large sharp-pointed tubercle in the form of an incipient nuchal spine. The 
extension of the occipital furrow along the posterior part of the cheeks is very 
deep and marked, giving high relief to the cheeks. The posterior rim is very 
narrow, but sharp and distinct, and the postero-lateral extremities of the head 
have very small acute terminations, without which they would appear some- 
what rounded. The cheeks are connected in front by a narrow, sharply rounded 
ridge, which lies a short distance from the glabella, along its anterior border. 
