88 WIIITEAVES ON THE ORTHOCERATID^ OF 



obtusely pointed posteriorly, thickest at the midleiigth, where it is very gently convex, 

 thence narrowing gradually and very slightly towards the aperture, which apparently is 

 simple and broadly truncated ; immediately b.'hind the aperture there is a faint annular 

 constriction ; siphoual and antisiphonal regions narrowly rounded ; sides compressed, 

 somewhat expanded, especially at the midlength and anteriorly ; chamber of habitation 

 occupying about one-third of the eutire length, which latter is more than twice but less 

 than three times the maximum breadth ; outline of transverse section, in the thickest part, 

 elliptical, with the longer axis of the ellipse not quite twice the length of the shorter; 

 surface showing indications of fine transverse costae, though the exterior of the test is not 

 very well preserved. Septa rather closely approximated and averaging about three milli- 

 metres apart ; siphuncle nearly marginal, moniliform, and slightly inflated between the 

 septa, the maximum breadth of each siphuncular segment being one-third less than its 

 height or depth. 



Approximate dimensions of the only specimen collected : length eighty-six milli- 

 metres ; greatest dorso-ventral diameter,* thirty-three mm. ; maximum lateral diameter, 

 nineteen mm. 



Black Island, near Swampy Island, Lake "Winnipeg, Messrs. Bowling and Lambe, 

 1890. 



In its broadest aspect, as represented by fig. 4, the posterior end of the specimen is 

 almost if not quite equilateral. The position of the siphuncle of this species was dis- 

 covered aud its characters ingeniously elucidated by Mr. Lambe, to whom the writer is 

 indebted for valuable assistance in working out the internal structure of some of the other 

 species described in this paper. 



Ottawa, December 5th, 1891. 



EXPLANATION OF PLATES. 



With the exception of figure 1 on Plate VI, and figures ], la and lb on Plate XI, all tlie figures are of natural size. 



PLA.TE V. 



Endoceras ANNULAÏUM. Var. (Page 77.) 



Fig. 1. — Side view of tlie most perfect specimen collected. 



la. — Outline of transverse section of the same, at the place indicated by the letter S on fig. 1. 



Endoceras subannulatum. (Page 78.) 



Fig. 2.— Posterior end of the largest and most perfect specimen collected. 

 2a.— Outline of transverse section of the same, at S, on fig. 2. 



* Assuming that the siphuncle is placed near the venter. 



