72 F. A. BATHER, CRINOIDEA OF GOTLAND. 



altérnate as usual. This type of ann-branching is precisely that shown in Heterocrinus 

 juvenis, ') and there can be little doubt but that the ancestral Calceocrmid had ."> arms 

 of this simple nature. We have already seen that the dorsal éup of Cästocriéus is essen- 

 tially the same as that of Hefieroermus; and thus the derivation of the Oaleeoerinidae from 

 a Heteroerinid ancestor, suggested by mc in May 1890, is fully coniirnaed. ? ) 



In conneetion with the varions changes that characterise the Calceocrinidse, this 

 simple and primitive arm-structure becomes variously mödified. In all members of that 

 family the right posterior arm has disappeared, its plaee heing completelv occupied by 

 the anal x and the following plates of the tnbe. Ho<w easily this may have come abont 

 is seen from the diagram of Heterocrinus bellevillensis given by Mr; Walter Billixgs, 3 ) 

 in which species »the firat anal plate .... rests principally on the second plate of the 

 right posterior series, but, slightly, on the first radial plate of the left posterior series.» 4 ) 

 It is not long before a similar fäte befalls the right anterior arm. The left anterior arm 

 gradually bifnrcates at a greater distance from the cnp and eventually ceases to braneh 

 altogether. We may therefore confine our attention to the arms that spring from the 

 large anterior and left posterior radials. The secondary bilateral symmetrv that is snper- 

 induced, in the eourse of evolution, on the crown of a Oalceocrinid affects these arms. s<> 

 (hat they form a right and left connterpart on either side of the ventral tube. The 

 following deseription refers to the anterior arm, and in it the terms 'right' and 'left' are 

 nsed as above, so that left' is adanal and 'right' is abanal or on the side of the median 

 (left anterior) arm. In deseribing the left posterior arm, the terms right and left wonld 

 simply have to be reserved in every case. 



In Castocrinus and in some species of Eiu-hjvocrinus the arm-branching only differa 

 from the simple type above described in the rather greater size of the right-hand division 

 of the arm. The armlets are probably simple as a rnle, but it is often, if not ahvavs 

 the ease that the first armlet of the left-hand division branches. This does not seem to 

 have becn noticed hitherto in Castocrinus, where this armlet is small; bnt in Euchirocrinus^ 

 vvhere the armlet has increased in size, the branching of it has been speeially described 



1>V Dr. RlNGUEBERG. 



Xo\v, so far as the arms are concerned, Caleeocrinus and Halysiocrinus differ from 

 the earlier genera chiefly in this, that the above-mentioned left (adanal) armlet is mneh 

 larg-er, and that it eontinnes to bifurcate on the left-hand ossicles. Each arm-division, 

 however, still branches on the same primitive plan. Therefore, though it is a general 

 rule that the extreme left-hand ossicles are all axillaries, we may still state the law of 

 branching as follows: running from right to left, each transverse series of the III, V, \ II 

 &c. Br. will be — armlet, axil, axil, armlet, armlet, axil, &c.; whilc each transverse series 

 of the l\ r , VI &c. Br. will be — axil, armlet, armlet, axil, axil &c. 



') Ann. Mag. Nat. Hist., ser. G, vol. V, Plate XV, lig. b b. 



-) In this conneetion it is interesting to remember that Mr. E. Billings — no niean authority actuallj 

 described two Calceocrinids as Heterocrinus incequalis and II. articitlosus. 



:l ) Ottawa Pield Nat. Club Träns. no. 4, p. 49 and plate: 1883. 



4 ) This little lat of deseription has been overlooked bj my learned friends Messrs. Wachsmutb and Sprin- 

 ger in their criticism of mj previous remarks on Heterocrinus; see The Perisomie Plates of the Crinoids Proc. 

 \r. Nat. Sci. Philadelpbia, vol. tor 1890, p. 383. 



