438 



DR F. A. BATHER. 



I" 5, and the choice lies between my interpretation of it as due to the fission of 

 18 or to the intercalation of an entirely fresh plate, and Dr Jaekel's original in- 

 terpretation of it as deltoid I'" 5 [ = 24]. The latter view is inconsistent with the 

 structures drawn in Jaekel's analysis of C. radiatus, since they already include 

 an V" b ; but stress should not be laid on that, for it is clear that the tegmen of 

 this species is too small and compressed to be studied with ease. In C. constrictus 

 the tegmen (text-fig. 44) consists of five deltoids in the normal position, and of these 

 the posterior, plate 23, is the widest. To accommodate this greater width, and to 

 receive the right posterior subvective groove with its numerous branches, the underlying 

 thecal plate is widened, and that plate is 18 [ = I" b\ which thus extends further beneath 

 the posterior deltoid than one might expect. There is no enlargement of deltoid 24. 



^ O'' ^^ O^' (3 



Fig. 44. 



Fig. 43. 



Text-fig. 43. — Analysis of Gheirocrinus constrictus. The invaginate portion of the basals is indicated by a 

 broken line. 



Text-fig. 44. — Diagram of the tegmen of Chcirocrinus constrictus, showing the true outline of the deltoids 

 (20-24). The radiodeltoid sutures are not shown in the diagram since their precise position has not been 

 satisfactorily made out, owing to the presence of the proliferated tegminal elements shown in 

 text-figs. 54, 55. Two of those elements are so large as to obtain representation in this diagram, to the 

 right and left of the posterior deltoid, 23. 



Consequently C. constrictus, at any rate, affords no reason for supposing that plate 24 

 could have been forced down into Circlet IV., but supports the suggestion that plate 18 

 may have widened still more and divided into two, the main brachioliferous portion 

 passing to the left beneath plate 23, as diagrammatically represented in my analysis 

 of C. penniget^ (1900 ; our text-fig. 40 his). 



In a word, the structure of C. constrictus supports my interpretation of 1900, 

 which, in the absence of such concrete evidence, I had felt inclined to give up. 



§ 297. Our knowledge of the species of Chcirocrinus does not as yet enable us 

 to state in how many the plate 1 8a is found, or from how many it is certainly absent. 

 It is therefore not possible to use this character for the subdivision of the genus. 



§ 298. The distribution of the pectinirhombs has only been recorded with 

 completeness for a few of the species, so that it is not easy to make general statements. 

 The assertion in the "Treatise on Zoology" (1900, p. 63), that rhombs 1-5, 1-6, with 



