MONOGRAPH OF THE EXISTING CRINOIDS. 325 



unabsorbed margins of the embryonic basal plates, is normally present, while at 

 the same time * * * the interradial processes of the rosette arc large and 

 spoutlike * * * and acquire a connection -with the remains of the primarj' or 

 dorsal layer which forms the basal bridge. The complicated rosette thus formed 

 becomes united Avith the large, more or less spindle-shaped rays of the basal star, 

 the origin of which is totally different from that of the rosette." 



Carpenter found that the rosette in Leptomctra lies much nearer the dorsal 

 surface of the radial pentagon than that of Antedon, and he also found that the 

 rosette of the species of ComasteridsB and of certain other oligophreate forms is 

 much better developed than that of these two genera. This was as much as he was 

 able to learn from the material at his command. 



In general the rosette of the Ohgophreata differs from that of the Macrophreata 

 in being more flattened, with the radial and interradial processes nearly on the same 

 pl^ne, and in being more developed, so that its total area is proportionately greater 

 and the 10 rays proportionately shorter and more specialized, the interradial 

 processes typically differing but slightly from the radial. It is also sunken consid- 

 erably below the level of the dorsal surface of the radial pentagon, having retreated 

 before the chambered organ and associated structures as they were shoved upward 

 by the increasing shallowness of the centrodorsal. The greatest departure from the 

 macropheate type is seen in the large comasterids, such as Comatella nigra, Comaster 

 novseguinese, Comactinia echinoptera and Comatula pectinata, though about the same 

 stage is seen in certain of the thalassometrids, as in Asterometra and in the Calome- 

 tridse. In many cases there has been such a development of calcareous tissue as 

 to conceal entirely the spaces between the 10 rays when the rosette is viewed in 

 position, while usually these are only visible as 10 shallow rounded notches, all of 

 equal size. 



The perfected state of the rosette in these forms is not acquired until the animals 

 are of their full size and development, the rosettes of the younger specimens being 

 more like those of the less specialized forms. 



The rosettes of the species of Zygometrida;, Thalassometridse and Tropio- 

 metridse usually have less developed interradial jirocesses than those of the species 

 of Comasteridfe, the gaps between the radial and interradial processes are deeper, 

 and the interradial processes curve inward (ventraUy) somewhat, so that they make 

 a slight angle with the plane occupied by the radial processes. 



In the species of Himerometridse, Stephanometridse, and Mariametridre the 

 rosette is still smaller, the gaps between the radial and the interradial i>rocesses 

 being deeper and broader, and the interradial processes are much more slender 

 than the radial, and curve upward at a considerable angle. The rosette of the 

 type most commonly seen in the Mariametridsc differs but little from that of 

 Antedon bifida and, as in that species, is usually but slightly sunk below the level 

 of the dorsal surface of the radial pentagon, giving evidence of the comparatively 

 close relationship between the more generalized mariametrids and the more 

 specialized antedonids such as those comj)osing the subfamily Antedonina\ evi- 

 dence which is in agreement with the deductions gathered from a study of other 

 characters. 



