202 On the Orthoceratites of Scotland. [March, 



still remained of a horny appearance, very thin, and well polished 

 within. 



2. 0. Pyra77]idalis.— Shell, tapering ; partitions, slightly waved; 

 chambers, large ; pipe, small and central. Fig. 2. 



This species bears a very close resemblance to the preceding in its 

 structure ; but the form of the shell is somewhat different, being 

 much longer in proportion to its breadth ; the length of the speci- 

 men is upwards of six inches ; the diameter at the largest end is 

 one inch and two- tenths, and at the apex two-tenths ; the last 

 formed chambers are nearly three-tenths of an inch in depth, v.'hile 

 the oldest, towards tlie point, are scarcely one-tenth. A fragment 

 of this species, found contiguous to the one described, and pro- 

 bably a portion of its inferior extremity, measured two inches in 

 diameter. 



S. 0. CyUndracea. — Shell, nearly cylindrical ; partitions, 

 slightly waved; chambers, numerous; pipe, minute and central. 

 Fig. 3. 



The largest specimen I possess is about three inches and a quarter 

 in length, four-tenths of an inch in diameter at the base, and 

 three-tenths at the apex ; the chambers are scarcely one-tenth in 

 depth. When the outer covering of the shell is removed, the 

 chambers appear very distinct, with a flat surface and a small groove 

 in the place of the partitions. 



4. 0. Convexa. — Shell, nearly cylindrical; partitions, thin and 

 convex ; pipe, large and lateral. Fig. 4. 



The length of this specimen is four inches and a half, the 

 diameter at the base one inch and a quarter, and at the apex 

 upwards of eight-tenths of an inch ; the partitions are remarkably 

 thin, and more convex than any of the preceding ; the lower 

 chambers are about an eighth of an inch in depth ; the pipe is 

 about three-tenths in diameter, and placed about half way between 

 the centre and the margin. 



5. 0. Atfemiata.—ShtM, tapering; partitions, nearly circular; 

 chambers, large. Fig. 5. 



The shell of this species, in one specimen, is very thin, trans- 

 parent, and glossy, and in some places it is minutely striated across. 

 The length of another is upwards of six-tenths, the breadth at the 

 base about one-eighth, and at the apex about one-twentieth, of an 

 inch. This space contains 15 chambers. 



B. — Surface of the Shell with Transverse Ridges. 



6. 0. Sulcata. — Shell, tapering ; ridges, waved and striated ; 

 pipe, small and central. Fig. 6. 



The length of the specimen is two inches and six-tenths, the 

 diameter at the base seven-tenths, and at the apex two-tenths, of 

 an inch. It contains 81 ridges, which are twice waved in going 

 round the shell. Both the ridges and intervening grooves are finely 

 ftriated. 



