104 DESCRIPTION OF THE HARD PARTS OF SOME 



The abactinal membrane of the disk of Brisinga is eminently Asterian ; 

 it is only slightly strengthened by a few minute limestone plates, as is 

 the case in Crossaster and Pycnopodia, and in spite of the general re- 

 semblance, at first glance, of this well-defined disk to an Ophiuran disk, 

 we have nothing whatever corresponding to the arrangement of the cen- 

 tral plates so characteristic of the disk of Ophiiirans. But we have in 

 a great many genera of Starfishes the central part of the disk, show- 

 ing in the young stages only, as regular an arrangement of the plates 

 of the abactinal system as in any Ophiuran, though it is lost in the 

 adult. Such a young stage is figured in PI. YIIL, a corresponding stage 

 has also been recently figured by Loven in his Memoir on the Echini 

 (1875), and a similar structure of the disk will undoubtedly be found to 

 exist in the very youngest stages of each genus, as it seems to be a 

 general structure of the young of all Starfishes, as far as observed. 



While Brisinga is a most important form, as showing the relationship 

 between Starfishes and Ophiurans, there certainly is nothing in its struc- 

 ture or in its affinity to Protaster to warrant the paloeontological importance 

 ascribed to it by the younger Sars ; and it cannot be considered, any more 

 than several other genera of Starfishes now living,* as the representative 

 at the present day of the oldest-known Echinoderm. I think we can 

 show from the study of the hard parts of Starfishes that they have been 

 a remarkably persistent type, and that the apparent changes of form 

 due to the excessive increase or diminution of the interbrachial lime- 

 stone deposit is a very secondary feature, which, though greatly modi- 

 fying the external appearance of the Starfishes, yet does not affect the 

 main structure, which, as has been stated, is remarkably uniform through- 

 out the order. Wliile fully admitting the many important points (so 

 well brought out by Sars in his Memoir on Brisinga) wherein the genus 

 differs from the other Starfishes, yet I must call his attention to tlie 

 fact that many of the structural details which he strongly insists upon 

 as special!}^ characteristic of Brisinga f are common to the other Starfislies, 

 and do not constitute features by which this family can be contrasted 

 with llic remaining Starfishes. 



♦ Pycnopodia, Crossaster. 



f For an oi)portinuty of examining both dry and alcoholic specimens of Brisinga, I must thank Sir 

 C. Wyvillc Thomson, and Dr. G. (). Sai-s. IJrisin<Ta cn<lccacnemos is found in dcop water off the Ix)foton 

 Islands, Norway. It h;vs been collected by the " Challenger" in eighty fathoms, on the La Have Bank olf 

 Nov I Scotia. 



