ASTEROIDEA. 



113 



almost all exposures. Up to the present our knowledge of the zonal occurrence 

 of these forms has been limited, as complete specimens of starfishes are exceed- 

 ingly rare. There appears to be no reason now, however, why our knowledge 

 of the zonal distribution of these forms should not become as nearly complete as 

 it is, for example, in the case of Echinoids. 



I must thank Dr. Blackmore, of Salisbury, for his invaluable suggestions to me 

 concerning this means of identification. 



6 



Text-fig. 3. — Isolated marginal of 

 Nymphaster Coombii, showing spines 

 and spine-pits. 



Ornament. — -The ornament of starfishes consists of calcareous pieces, Avhich 

 may be spinous in form, or scaly, or granular. These may occur : 



(!) Embedded in the living tissues outside the general body-plates, but not in 

 contact with the plates themselves. On the death of the animal they become 

 dispersed on the disintegration of the living tissues, and such ornament is therefore 

 rarely visible in fossil specimens. 



(2) Articulated to the plates. In this case they are situated : 



(it) Either in a depression of the plate ; 

 (b) Or in a depression upon a raised eminence 

 of the plate. Occasionally in this latter case 

 the depression may be excavate in the centre 

 in order to allow a strong muscular attach- 

 ment. In this case the eminence may 

 simulate the perforate tubercle of an Echi- 

 noid such as Cidaris. 

 In almost all cases in Cretaceous Asteroids the 

 ornament is of the type 2 a. 



Generally the movable articulated pieces have 

 disappeared, but in such cases the depression on 

 the plate which they formerly occupied is readily 

 visible (compare Text-fig. 3). 



I purpose to call all such movable articulated 

 pieces, whether they are spinous or granular in 

 character, " spines," and, at the suggestion of 

 Dr. Bather, the depressions on the plate " spine-pits." 

 The character of the spine-pits appears to be 

 constant in character in each individual species. 

 Thus, e. g. in iV. Coombii (Text-fig. 3) they show a coarse honeycomb structure, 

 uniform in character over the whole of the plate. In Stauranderaster bulbiferus 

 (Text-fig. 4) the spine-pits are circular and widely spaced. This latter type is 

 interesting, as it apparently occurs only in the genera Metopaster and Stauran- 

 deraster. The spine is very small, and barely projects over the edge of the deep 



Text-fig. 4. — Isolated marginal of 

 Stauranderaster bulbiferus, showing 

 the " pustulate " type of ornament. 



