448 



DR F. A. BATHER. 



§ 333. Measurements of Cheirocrinus constrictus in millimetres :- 





G157. 



G188. 



G133. 



G152. 



G166. 



Height of theca 



21-5 



22-8 



22 



23-7 



21 



Width at Circlet H. 



18 



17-7 



17-2 



17 



16 



Width at top of Circlet III. . 



14 



ca. 11 



ca. 9 



12-5 



12-5 



Width of tegmen . 



177 



ca. 14 



13-7 



13-5 



14 



Length of stem 



39 



32 



30 



35 



32 



Proximal diameter stem . 



5-2 



4-8 



5 



4-6 



5-8 



Distal diameter stem 



1-5 



1-5 



1-5 



M 



1-6 



§ 334. Relations of the Species. — In the conjunct pectinirhombs C. constrictus 

 agrees only with C. giganteus and C. atavus, but differs from them and from all other 

 species of the genus in the restriction of the basal rhombs to 1-5. Pectinirhomb 1-6 

 is not shown in several of Barrande's figures of C. alter, but Dr Jaekel definitely says 

 "2 basale Rauten." Apart from this character, C. constrictus difi"ers from those three 

 species in its more invaginate base. 



§ 335. The remarkably regular arrangement of the thecal plates and the simple 

 conjunct structure of the pectinirhombs are most naturally regarded as characters 

 retained from an ancestral form. But the suppression of many pectinirhombs, notably 

 in Circlet I., the invagination of the base, and the constriction of the theca in correla- 

 tion with the expansion of the tegmen are, in my opinion, signs of definite, though not 

 excessive, modification. It may be suggested that the conjunct nature of the pectini- 

 rhombs is correlated with their reduction in number ; for it is clear that a few conjunct 

 rhombs may aff'ord the same functional area as a much larger number of disjunct 

 rhombs (cf. C. interruptus, §§ 346, 347). 



§ 336. Cheirocrinus interruptus. 

 (PL IV. fig. 48. Text-fig. 59.) 



1899. Ghirocrinus interruptus O. Jaekel, " Stammesgesch. d. Pelmat.," pp. 204,217, 220, 

 pi. 10, f. 9. 



§ 337. Diagnosis. — A Cheirocrinus with numerous complete pectinirhombs, each 

 multidisjunct ; thecal plates with narrow, strongly marked axial and radiating ridges, 

 which sometimes traverse the pectinirhombs ; also with isolated fine radiate rows of 

 granules. 



§ 338. Syntypes, two, in the St. Petersburg Academy. Theca with a diameter 

 circa 15 mm. 



§ 339. Locality. — The syntypes are said to be from the neighbourhood of St. 

 Petersburg. The present specimen is from Thraive Glen, Girvan. 



§ 340. Horizon. — That of the syntypes is not precisely known ; the occurrence 

 of the present specimen in the Starfish Bed would naturally have suggested the 



