211 



dorsal and ventral regions compressed, though perhaps abnormiiUy so, 

 the outline of a transverse section through the thickest part, neaii' to the 

 body chamber, being broadly elliptical. Septate portion, in the only 

 specimen known to the writer, occupying about two-thirds of the entire 

 length, and divided into seventeen chambers, its apical extremity 

 obtusely pointed ; chamber of habitation nearly cylindrical, though its 

 maximum diameter is about six millimetres greater at its commencement 

 posteriorly than at the aperture. Surface markings unknown. Septa 

 shallowly concave externally in the dorsal and ventral regions, their 

 distance apart averaging about one-third the maximum diameter, except 

 the two or three last formed, which are rather closer together. Siphun- 

 cle, as seen in a longitudinal section of the specimen, eccentric, sub- 

 marginal, very large and much swollen between the septa posteriorly, 

 but ultimately much diminishing in size towards the chamber of 

 habitation. At the apex, posteriorly, nearly the whole of the first 

 chamber is filled up with the first segment of the siphuncle, which is 

 twelve millimetres in its maximum diameter. In the next five or six 

 chambers, the siphuncular segments fill the greater part of the space, 

 and average from fifteen to sixteen millimetres in their maximum 

 diameter, after which the siphuncle decreases so rapidly in size that in 

 the fourth chamber from the body chamber, the maximum diameter of 

 the segment therein contained is only five mm. Dimensions of the 

 specimen figured and described ; total length, about nine inches, or 23 

 cm.; greatest thickness, 36 mm. ; length of the septate portion of the 

 shell, as measured in the median line of the section, 14"6 cm. ; greatest 

 diameter of the body chamber, 35 mm. posteriorly and 29 mm. anteriorly.'' 



Little Black Island, Lake Winnipeg, J. B. Tyrrell, 1889 : one nearly 

 perfect cast of the interior of the shell, with the whole of the siphuncle 

 and septa well preserved. 



" The internal structure of this species is essentially similar to that of 

 the Ortlioceras docens of Barrande, which Professor Hyatt regards as an 

 aberrant member of his genus Sactoceras, but the external contour of 

 these two shells seem to be quite different. The remarks which Professor 

 Hyatt makes upon >S^. docens, however, are quite as applicable to the 

 Canadian species. In reference to the former, he states that ' it is a 

 transition form ' but we place it in this genus because at an age when 

 an Actinoceras would have the rosettes large and perfect, this species 

 begins to lose them, and the siphon decreases also. The reduction of the 

 siphon is a degradational senile shrinkage, and it occasions the loss of the 

 rosettes. M. Barrande views this old stage of the siphon as a return to 

 the tubular siphon, but in our opinion we cannot call this a tubular 

 siphon. As a matter of fact it is a modified nummuloidal siphon, as may 

 be seen by comparison with others." 



6J 



