V. Botryocrinus. 409 



■will be remembered, was the case with Thenar ocrinus. The 

 pentagonal outline of the &tem-lumen, even when radial 

 sutures are no longer visible externally, seems, in other genera 

 no less than in Botryocrinus, to bear witness, often in a marked 

 manner, to tlie former pentamerous structure of the stem- 

 ossicles. Whether this pentamerous structure is really more 

 primitive or ancestral than a simple undivided ossicle, espe- 

 cially an ossicle with a round lumen, we are not at present in 

 a position to decide. There are, however, two facts, a con- 

 sideration of which may eventually aid us in solving the 

 problem. First the fact that the radicular cirri in this genus, 

 and indeed all cirri in every genus where longitudinal sutures 

 or their traces are present, arise along the suture-line. The 

 morphological importance of this fact, as denoting the radial 

 orinterradial position of the original sutures, has already been 

 noticed by Messrs. Wachsmuth and Springer in connexion 

 with the presence or absence of infrabasals. Secondly the 

 fact that towards the distal, i. e. the first-formed part of the 

 stem, the pentameres of any one ossicle, instead of all lying 

 in the same plane, abut against the pentameres of adjacent 

 ossicles, and are only brought into line by the curvature of 

 their articular surfaces. The outline of each pentamere has 

 in fact six sides, and the pentameres have an essentially 

 alternate arrangement just like the hexagonal plates of a 

 ventral sac (fig. 5, p. 408). 



Inter-relations of the Species. — There can be little doubt 

 but that, in accordance with the main scheme of arm-evolution 

 sketched in Paper II. (' Annals,' ser. 6, v. p. 374), the pinnu- 

 late forms are descended from those bearing arndets. Of the 

 armlet-bearing forms there are three species, B. ramosus in 

 England, B. 4'umossimus and B. cucurhitaceus in Gotland. 

 Oi these B. ramosus is very closely allied to B. ramosissimus. 

 If Professor Lindstiom's correlation of the Gotland beds * be 

 correct, B. ramosissimus appears at or above the horizon of 

 the Lower Ludlow (/), and might therefore be directly 

 descended from B. ramosus of the Upper Wenlock. B. cucur- 

 bitaceus, however, occurs in beds of the age of the Wenlock 

 shale (c), and the intermediate links are not known, for no 

 species of Botryocrinus are yet recorded from beds d and e. 

 It may have been that B.. ramosus and the other English 

 forms were derived from some of these links. Taking the 

 English forms, there seems little doubt but that the order in 

 which we have studied them is the order in which they were 

 evolved. We have traced the change of armlets into pinnules, 



* Prof. G. Lindstrom, " Ueber die Schichtenfolge des Siliir auf der 

 Insel Gotland," Neues Jnhib. f. Min., Jahrg. 1888, Bd. i. pp. 147-164. 



