CARADOCIAN CYSTIDEA FROM GIRVAN. 



437 



Jaekel's analysis, in which the sign l"'b [ = 24] is affixed to the plate that 1 have 

 numbered 18a. If Dr Jaekel's diagram were a correct statement of fact, then this 

 interpretation of his would seem superior to mine. Unfortunately, on pp. 201 and 213, 

 he himself expresses doubts as to the correctness of the drawing, and, on the evidence 

 of better material, suggests that his plate I" b [= 18] is really V"l, that is to say, the 

 posterior deltoid 23, and that his plate l"'b [= 18a] is really l"b [=18]. This would 

 do away with his I'" \ [=23] or reduce it to the status of a side-plate, and it would 

 provide nothing to take the place of deltoid 24, the true I'" b. Dr Jaekel's original 

 interpretation certainly is easier to understand. 



§ 294. Turn now to Dr Jaekel's analysis of C. radiatus (1899, p. 213, f 40 ; our 

 text-fig. 42). Here a similar difficulty meets us. The plates follow regularly till we 

 reach a plate (lettered Z'"l = 23) occupying the position of 18 in my diagram. 

 Between this and plate 19 is a quadrangular plate (lettered I" 5 = 18), resting on 13 



Text-fig. 42. — Analysis of Cheiroerinus radiatus, after Jaekel (1899). This diagram 

 has been constructed in the same way as text-fig. 41, 



and corresponding to my 18a. From this diagram it is quite impossible to imagine 

 how the tegmen was constructed. But there are now other reasons for doubting its 

 accuracy and its interpretation. 



§ 295. The abundant material of Cheiroerinus constrictus has enabled me to present 

 an analysis (text-fig. 43), of which every essential element can be demonstrated. In 

 this there can be no question as to the interpretation of the five plates in Circlet IV. ; 

 obviously they are plates 15-19, the Radiolateralia l"l-l"b of Jaekel. The form 

 and relations of plate 18 in particular are perfectly clear, and its homology with 

 plate 18 in my analysis of C. penniger [IdQQ) is self-evident. That plate, therefore, 

 is, as Dr Jaekel originally supposed, a radiolateral l"b ; and the same indubitably is 

 the case with the corresponding plate in C radiatus. On this point disagreement 

 no longer appears possible. 



§ 296. Cheiroerinus constrictus, however, has no thecal element corresponding to 

 my plate 18a, i.e. l"b [=18] in Jaekel's analysis of C. radiatus. What, then, is 

 that additional element ? It is no longer possible to regard it as a radiolateral 



