ASTEEOIDEA OP H.M.S. 'cHALLEN&ER' EXPEDITION. 191 



three new genera, viz. : — Marsipaster, two species ; Benthaster, 

 two ; and Calyptraster, one. 



Note on Terminology. — For the sake of brevity aad to avoid 

 verbose repetition, several terms are employed in the following- 

 descriptions which have not previously been used in their present 

 special signiiication. The introduction of these terms is necessi- 

 tated by structural peculiarities iu the forms comprised in the 

 family Pterasteridse, several of which have hitherto been unob- 

 served, whilst others have been ignored or passed over by previous 

 systematists. The application of the terms will, in most cases, be 

 self-evident. The following is a brief definition. 



The suj} ret dorsal membrane is the veil-like^ covering or external 

 independent tissue whereby the dorsal nidamental cavity is 

 formed. The membrane is supported above the true dorsal sur- 

 face of the animal by the paseillce, which consist of a long columnar 

 pedicel surmounted by a "crown" of fine, more or less elongate 

 spinelets. In the majority of forms belonging to this family, 

 fine muscular fibrous bands extend between the tips of the spine- 

 lets, and constitute a more or less regular fibrous network ; and 

 the general tissue of the dorsal membrane which fills in the inter- 

 spaces or meshes is usually perforated by small contractile pores, 

 styled spiracula by Sars. A large aperture occurs in the supra- 

 dorsal membrane, situated over the centre of the disk and opening 

 directly into the dorsal cavity, to which it aft'ords the common 

 means of ingress and egress ; it is named the oscular orifice. 

 This aperture may be closed by five more or less regular fan-like 

 valves, or simply by a number of webbed or papillose spinelets. A 

 number of small apertures open into the nidamental cavity on the 

 actinal surface of the Starfish, an aperture being situated at the 

 base of each of the long actino-lateral spines and close up to the 

 adambulacral plate. There is consequently an opening into the 

 cavity on either side of the furrow corresponding with each seg- 

 ment of the ray ; hence these are spoken of as segmental aper- 

 tures. The openings are guarded, and can be closed, by a small 

 spinelet or papilla articulated on tlie adambulacral plate, and 

 termed the apeo'ture-papilla. In some genera these appendages 

 are partially hidden in the actinal membrane, and are free on 

 one side only ; in others they are perfectly free, and covered 

 with a more or less expansive investing membrane of their own. 

 The long spines articulated on the body-frame close to the adam- 

 bulacral plates, and which form the lateral or marginal web iu 



