KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 25. N:0 2. 71 



that the orals, or whatever tegminal plates existed, practically helped to form the anterior 

 wall of the tube. 



5. Anal Tube. Messrs. Wachsmuth and Springer 1 ) describe the anal or ventral 

 tube as composed posteriorly of a series of large quadrangular plates, »the anterior side 

 being composed of ven - small and delicate plates». Dr. Ringueberg (qp. eit., p. 391) gives 

 this last statement on the authority of Wachsmuth and Springer, and does not seem to 

 have seen the structure himself. It is partly eonfirmed by the best specimen of C. pinnu- 

 latus (see p. 97); but here, in the distal part of the tube, the distinction between the 

 posterior series and the smaller anterior plates is not so great. Further, the tube, instead 

 of »extending to the tips of the arms and perhaps beyond their limits, with scarcely any 

 diminution in size», becomes markedly smaller above. This seems to have been the case 

 in all European species of Calceocrinus, and it is doubtful whether the tube often extended 

 beyond the arms: it was probably only exposed when the crown was erected and the arms 

 opened; there would then be a broad fan of arms and pinnules, unfolded equally on either 

 side of the left anterior radius, from which fan the ventral tube would project at a con- 

 siderable angle. There is most remarkable resemblance between the large outer plates of 

 the tube and brachials, as shown in a section of a crown of C. tenax (Pl. III, tig. 111). 



6. Arms and Pinnules. In the descriptions of Calceocrinidae given by various 

 authors, the term »pinnules has been very loosely used. Here it will be used in the 

 sense ascribed to it in mv paper on »The Classitication of the Inadunata Fistulata»: 2 ) 



When finally the armlets become small, cease to branch, and are regularly placed on 

 alternate sides of successive joints, they are called Pinnules.» In this sense it does not 

 appear that pinnules have yet been described for any Calceocrinid. The minor branches 

 of Castocrinus furcillatus, for instance, which Ringueberg calls pinnules, are given off 

 from every third ossicle. It is probable too that, in many cases, these so called pinnules 

 really branch again; in fact by many authors the term has actually been applied to what 

 is, as we shall see, morphologically the major portion of the arm. The truth of this 

 statement is proved by the Gotland species, which present us with an exceedingly interest- 

 ing evolutionarv series showing the gradual differentiation of the arms. 



Before this evolution can be understood it is necessary to grasp the law that governs 

 the branching of the arms in most, if not all, of the Calceocrinida\ And first let us 

 consider what was the type of arm-branching in the ancestral Calceocrinid. Examination 

 of the four arms of Castocrinus fureittatus shows that each arm is constructed on exactly 

 the same plan: if any one of these arms be considered separately, without reference to 

 the orientation of the body, if it be placed with the distal end upperinost, as is usual, 

 and if the terms 'right' and 'left' be used with reference to the right and left of the ob- 

 server, then this plan may be described as follows. The arm bifurcates on the second 

 primibrach, and in each division an armlet is given off from every 3rd or 4th ossicle. 

 The first or proximal armlets are given off on the outer sides of the arm, i. e. on the 

 right of the right-hand branch, on the left of the left-hand branch: succeeding armlets 



') Revision IIJ, 281 Proc. 188G, p. 205. 



2 ) Ann. Mag;. Nat. Hist., ser. 0. vol. V, p. 374. May 1890. 



