1916] The Ottawa Naturalist 109 



strands at right angles to the sutures between the thecal plates. In 

 order to bring Comarocystites in line with pseudo-pentamerous 

 Rhombifera, the former presence of an anterior ray of the food-groove 

 system is imagined. 



Jaekel, in 1900, separated from the Cystidea, under the name 

 Carpoidea, a considerable number of the genera included by Haeckel 

 in his Amphoridea, adding also the genera Malocystites, Canadocy- 

 stites, and Amygdalocystites, included by Haeckel under his Cystidea, 

 in the restricted sense. The chief characteristics of the Carpoidea were 

 supposed to be: a loose relation of the ambulacral organs to the theca- 

 leaving only slight traces on the latter; theca never pentamerous, often 

 distorted, usually compressed dorso-ventrally, more or less symmetrical 

 toward the right and left; ambulacra extending into two radii; the 

 brachials bearing the ambulacral grooves uniserial as far as known; 

 base tetramerous or trimerous Those Carpoidea possessing biserial 

 columnals Jaekel placed in the subdivision Heterostelea, and those 

 possessing a single series of ring-shaped columnals he placed in the 

 subdivision Eustelea. The Eustelea included Malocystites* Canado- 

 cystitis, Amygdalocystites, and Comarocystites. 



It must be acknowledged that the four genera here listed form a 

 very coherent group in which trimerism or pseudo-pentamerism seems 

 never to have prevailed. Under Bather's term, Malocystidae, this 

 group has been placed among the Amphoridea in the more recent 

 editions of Zittel. The relationship between Canadocystis, Amygdalo- 

 cystis, and Comarocystites appears especially close. All of these forms 

 are bisymmetric with the main apical food-groove extending laterally 

 from the mouth, the anal pyramid being on the right side of the theca. 

 Both the brachials and pinnulars are arranged in uniserial order. 

 When the arms are oriented so that the ventral side faces away from 

 the observer and the distal side of the arm points upward, then, in 

 all three genera, the pinnules are seen to form a single row on the right 

 side of the arms. In Comarocystites the arms are free. In Amygdalo- 

 cystites and Canadocystis the arms are twisted in contrasolar direction 

 and are attached by their left sides to the theca, leaving the right side 

 free for the pinnules. 



In the structure of their thecal plates, however, all three genera 

 differ greatly. In Comarocystites, the vertical plates of the meso- 

 stereom, as exposed on the inner side of the theca, suggest strongly the 

 plates characterizing the pectin irhombs of the Rhombifera, although 

 the spaces between these plates do not open at the top in slit-like pores, 

 as in true pectinirhombs. In Amygdalocystites, the inner surface of 

 the thecal plates is marked by radial ridges- which in some specimens 

 are sufficiently defined to be called short plates. One radial ridge 

 always extends to each of the angles of the plate, and in some specimens 

 another ridge extends to the middle point of each side. In some 



