8 BRITISH PALEOZOIC ASTEROZOA. 



different in strnctnre from the true Asteroidea. In this monograph Schondorf's 

 argument will l)e further extended, and it Avill be shown that true Asteroidea are 

 rare fossils in Palaeozoic rocks, at any rate in Ordovician and Silurian strata. 



(b) 21ie General Cdlcificatlon of the Body. — Observations of the general calcifi- 

 cation of the body can also be most misleading. The methods of calcification are 

 much more diverse than were imagined by the earlier observers, who only looked 

 for structures found in Recent groups. In consequence, considerable revision of 

 our fundamental conceptions has become necessary. 



Apparently the first ossicles to be laid down were those associated with the 

 water-vascular system (ossicles of the ambulacral groove and mouth-frame). The 

 general body-skin remained uncalcified. 



A leathery body-skin with few oi' no calcifications can be seen preserved on 

 the pyritised specimens found by Stiirtz in the Bundenbach (Lower Devonian) 

 slates. Under less favourable conditions of preservation all traces of the skin are 

 lost. Certain forms, e.g. Tseniaster, Billings, must have been in this primitive 

 condition, for no plates except those connected with the ambulacral groove and 

 mouth-frame have been found. The sandstone and limestone rocks in which they 

 are found had such a coarse matrix that the leathery skin could leave no impres- 

 sion on its surface. 



Later the leathery skin became protected by calcifications. Openings are 

 usually left between the plates so that some portions of the skin may be left 

 exposed to the surrounding sea-water, for the skin is the main lung of the 

 starfish. 



The manner in which calcification proceeds appears to be highly characteristic 

 of the various branches. Thus in the Asteroid branch, a double row of marginalia 

 begins to become larger than the remaining plates of the disc in the Uranasteridge 

 (Text-fig. 20, p. 23). These marginalia increase in size, and the disc tends to become 

 so heavily plated that the animal stands in peril of becoming choked by its too 

 complete armour. New modifications, the Cryptozonia, arise from these Phan- 

 erozonate forms. In these later types the ossicles are cut away so that as much as 

 possible of the skin may be exposed as a lung. Additional protection is given by 

 modified spines, the pedicellaria, which sit upon the plates and seize any small 

 animal which disturbs the luno- outo-rowths of the skin. 



Branches whiclb are noio extinct had their own method of calcification. Thus 

 the Aspidosomatidje have been shown by Schcindorf to have had but a single row 

 of marginalia arranged in a highly characteristic manner. 



I propose, in later parts of this monograph, to follow out these divergent 

 methods of calcification very closely. My observations (74) upon the evolution of 

 the Cretaceous Asteroidea taught me that the only satisfactory way to separate the 

 various lineages was to pay attention to the ornament and shape of the plates of 

 those parts of the skeleton which w^ere not considerably affected by the environ- 



