220 JRemeics — Wachsmuth and Sprmger — On Crihoids. 



'brachials, and (&) the Actinal plates, developed on the left antimere 

 and connected with the mouth, viz. orals and all plates of the 

 ambulacra. The secondary or supplementary elements are all the 

 interradial, interbrachial and interambulacral plates, including the 

 anal plates and plates of the tube or sac. These apparently are what 

 they call " perisomic," although the term would seem legitimately 

 applicable only to plates that, one way or another, form part of 

 the calyx. 



The title, however, in no way does justice to the contents of the 

 paper. For the authors discuss the homologies first of the plates 

 of the ventral surface generally, secondly of the anal plates ; and as 

 side-issues, but important ones, they treat of the interradial plates 

 and of the large plates, hitherto called interradials, seen in the 

 tegraen of the Cyathocrinidas. 



Those who have regarded the question in a purely morphological 

 aspect have long doubted the existence, at least in many cases, of 

 a " vault," by which was meant that outer roof of plates supposed, 

 in the Camerata, to conceal another integument, often plated, homo- 

 logous with the so-called " disk " of an Antedon. Thus Dr. P. H. 

 Carpenter in his " Challenger " Eeport on the Stalked Crinoids 

 (1884), although he still allowed a "vault" to the Actinocrinidse, 

 could not accept it for the Reteocrinidge, Ichthyocrinidee, and most 

 of the Platycrinidge. The admission of Messrs. Wachsmuth and 

 Springer (Revision III.) that the calyx interradials were continuous 

 with those of the "vault" seemed inconsistent with the presence of 

 other interradials in a supposed " disk " beneath the " vault " ; the 

 growth of plates outward and downward from the orals and upward 

 from the interradials, through regions where no connective tissue 

 previously existed, the two meeting in air like a cantilever bridge, 

 was incomprehensible; while the grave statement that the plates 

 of such a " vault " corresponded one for one with the plates of 

 the underlying " disk " (Revision III. 60) showed the untenability 

 of the whole hypothesis. There were — as there are still — certain 

 difficulties in any other explanation ; but the paper of Messrs. 

 Wachsmuth and Springer on Taxacrinus, etc., furnished so complete 

 a set of links between the " disk " and the " vault " that doubt was 

 no longer possible, and in April, 1890, the present writer denied 

 the existence of any structure covering the disk, and proposed in all 

 cases to use the word tegmen (Ann. Mag. Nat. Hist. [6] v. p. 313). 



Messrs. Wachsmuth and Springer have now confirmed this con- 

 clusion by finding satisfactory homologies for the plates of the 

 tegmen in Actinocrinidse. The simplest form of tegmen in Crinoids 

 consists of only five plates, interradially disposed, that meet over 

 the mouth and are called orals : this is now regarded as also the 

 earliest form. It is supposed that, to increase the calycal cavity, 

 interradial plates were developed between the radials, while similar 

 interambulacral plates, as well as extensions of the ambulacral plates, 

 gradually intervened between orals and radials. The orals might 

 disappear or remain. As the lower parts of the ai'ms became in- 

 corporated in the dorsal cup, more and more ambulacral and inter- 



