1916] The Ottawa Naturalist 73 



by the lateral junction of pairs of uniserial arms, but rather to 

 call attention to the fact that the arms of certain cystids appar- 

 ently present similar problems. Since these cystids are not as 

 fully known as desirable, a more detailed description of Coma- 

 rocysiitcs is given here, and a few notes on Caryocrinites are 

 appended. Moreover, these are the only cystids known at pre- 

 sent in which the arms are free and pinnuliferous, and, as such, 

 possess special interest. Both genera are American, occuring 

 both in Canada and in the United States. 



II. Detailed Description of Comarocystites puxcta- 

 tus Billixgs. 



5. Chief characteristics of the theca. Theca obovate, some- 

 times attaining a length of 75 millimeters, composed of about 

 150 plates, most of which are hexagonal in outline. Theca 

 moderately compressed from front to rear. The two primary 

 food-grooves diverge toward the right and left from the mouth 

 in such a manner as to present the appearance of a single trans- 

 verse, slightly curved, food-groove (Plate II, figs. 1A, IB). The 

 mouth does not present the appearance of a slit, as in Aristo- 

 cystis bohemicus Barrande, and apparently also in Caryocystis 

 angelini Haeckel, but takes the form of a more or less circular 

 or elliptical aperture located in the bottom of the transverse 

 apical food-groove already described. At each end of this food- 

 groove the latter branches dichotomously on the proximal side 

 of a nodular protuberance of stereom about 10 or 11 millimeters 

 in diameter. Each nodular protuberance supports two arms. 

 There are, therefore, four arms, arranged in pairs, one pair at 

 each end of the transverse apical food-groove. These corres- 

 pond in position to the lateral arms of the five-rayed cystids, 

 there being no arm corresponding to the anterior arm of other 

 cystids. The anal pyramid (Plate II, figs. 1A, IB, 2; also Plate 

 III) is situated a short distance below the protuberance support- 

 ing the pair of arms on the right side of the specimen. In larger 

 specimens the transverse apical food-groove, between the points 

 of dichotomous branching, has a length of about 13 millimeters, 

 thus giving to each of the two lateral primary rays a length of 

 6 millimeters. Throughout its length the transverse apical 

 food-groove follows the suture line between the anterior and 

 posterior peristomial thecal plates. Along the basal margin 

 of the nodular stereom protuberance, the e cterior surface of 

 the adjoining thecal plates of some specimens presents the 

 appearance of being crowded back by the growth of the pro- 

 tuberance, and consequenlty of being reduced in size. The 

 upper margin of these thecal plates appears to rest against the 

 lower half of the protuberance, but cross-sections of other 

 spfvnmens indicate that the upper inner margin of these thecal 



