72 



CATALOGUE OF THE BLASTOIDEA. 



Goette 1 that the oral plates of a Neocrinoid are truly homologous with the basals, 

 being developed spirally round the left peritoneal sac, while the basals appear in a 

 similar spiral round the right peritoneal sac; and from the year 1879 onward, the 

 homology of the proximal dome-plates with the orals of a Neocrinoid has been steadily 

 insisted on by one of us. 



The simplest form of vault which occurs among the Palseocrinoids is that presented 

 by the young stages of Allagecrinus. We have examined a large series of specimens 

 of this type, but have never been able to make out that the vault consists of anything 

 more than the five plates which we have described as orals 2 . In Haplocrinus the 

 apex of the vault is capped by an orocentral, which reaches a relatively larger size in 

 Symbathocrinus. Neither Allagecrinus nor Haplocrinus has any calyx-interradials, 

 and the oral pyramid, as we regard it, rests directly upon the radials. In Culico- 

 crinus, however, the five large plates which form the vault rest on the calyx- 

 interradials, the posterior one being rather the larger, with its distal margin excavated 

 for the anal opening. In this type the orocentral seems to be absent, as in Allage- 

 crinus; but in Platycriiius there is rather more complexity of structure. Besides 

 the orocentral and the proximals, there are some other plates in the vault which 



Fig. VI. B . 



Diagrams to illustrate the relation of the summit-plates to the interradials of the abactinal 



system. 

 A. EUvacrinus elegans (modified from Hall). B. Platycriuus, sp. 3. Basals. 4. Radials. 

 /. Intenadials. 0. Proximal summit-plates (Orals). 



bound the ambulacral openings, and the posterior proximal seems to be represented 

 by two plates which are separated by the anal tube. In the simplest form of 

 Tlatycrinus the proximals rest directly on the primary interradials of the calyx 

 (Fig. VI. B). But in other species, and in the Actinocrinidte, the structure of the 

 i -■ Vergleichende Entwickelungsgeschichte der Comatula Mcditorranca." Archiv f. Mikroskop. Anat. 

 L876, Bd. xii. pp. 595, 621. 



■iii, Allagecrinus, the ltepresentativc of a new Family from the Carboniferous Limestone Scries 

 ,i g otiand." Aim. & Mag. Nat. Hist. 1881, vol. vii. pp. 285-287. 



