ME, P. H. CAEPEXTEE ON THE GENUS ACTIXOMETEA. 107 



are formed almost entirely by what was once a stem-segment ; while in Pentacrinus this 

 cavity is a part of the central space enclosed within the radial and basal pentagons, which 

 respectively form the ventral and dorsal portions of its side walls. Among the fossil 

 Apiocrinklce we find an intermediate condition between these two extremes. Thus in Ap. 

 mespiUformis, as seen in Goldfuss's figure 1 , the first radials are small, but the basals are 

 very large and curved outwards, so as to enclose a large central cavity ; this we may 

 fairly suppose to have lodged a chambered organ, as Ludwig 2 has found that in Rhizo- 

 crmus, the modern representative of this family, the axis of the stem expands into a 

 chambered organ just as described above in Pentacrinus. I have found the same to be 

 the case in Bathijcrinus. 



This organ is contained in H. 1 of of easts in a large and apparently simple segment, 

 described by Sars 3 as the expanded uppermost stem-segment. Pourtales 4 , however, for 

 reasons which will be discussed further on, regards it as composite and as representing 

 the five basals. In this case the relative position of the chambered organ is precisely 

 identical with that which we may suppose it to have occupied in Ap. mespiUformis, 

 namely, on the dorsal side of the radial pentagon, but not within the uppermost stem- 

 segment, as in Comidula. In Ap. rosacens 5 the relative position of the chambered organ 

 must have been very much as in Pentacrinus, though slightly higher; for the cavity in 

 which it was contained was almost entirely enclosed between the enlarged first radials, 

 while the basals only form its floor and the very lowest portion of its side walls. Lastly, 

 in Ap. Ililleri 6 the chambered organ must have lain altogether on the ventral side of the 

 basals; the superior surfaces of which form by their apposition the floor of a cavity whose 

 side walls are entirely composed of the adjacent inner faces of the contiguous first radials. 



This condition is thus precisely the opposite of that which we find in Comatula, where 

 the cavity containing the chambered organ is not only altogether outside the radial 

 pentagon, but also on the dorsal side of the rosette or metamorphosed basals. 



(§ 73) In the works both of Goldfuss and Miller may be found incidental suggestions 

 that the " basis," or circlet of basals, and, indeed, the whole of the lower part of the calyx 

 of the stalked Crinoids, may be regarded as representing expanded stem-segments, each 

 broken up into five parts. Muller, however, was the first to put this idea into a definite 

 form. He described the basals of Pentacrinus as a metamorphosed stem-segment 7 , or as 

 " zerfallene Theile eines obersten Stengelgliedes ;" for they correspond in position with 

 the five leaf-like figures on the articular surfaces of the stem-segments which mark the 

 positions of the five longitudinal tendons. 



The fibrous bundles composing these tendinous cords are separated from one another 

 by a very regular calcareous network, which is deposited around and between them, 

 somewhat as in the rays of the basal star of Actinometra. They are attached (by their 



1 Petref. Germ., Taf. lvii. fig. 1, n. 



' " Zur Anatomie des li. lofotensis," Zeitschr. f. wiss. Zool. Ed. xxix. p. 1:2:2. 



3 Crinoides vivants, loc. tit. pp. -4, 12. 



4 " On a new Species of Bhizocrinus from Barbadoes,'' Zoological Results of the Hassler Expedition, p. 28. Cam- 

 bridge, U. S., 1374. 5 Petref. Germ., Taf. lvi. fig. 3, e. 



6 Petref. Germ., Taf. lvii. fig. 2, d. 7 " Bau des Pentacrinus" loc. eit. pp. 10, 25. 



SECOND SERIES. — ZOOLOGY, VOL. II. 15 



