LIFE IN PRIMARY PERIOD 223 



individuals ; these primary forms, however, were very 

 unlike those of the present day and sometimes not so clearly 

 differentiated. For instance, there were Star-fish with the 

 madreporic plate situated on the ventral side of the body, 

 as in the Ophiuroids. In the Sea-urchins, the ambulacral 

 areas were very narrow and furnished with enormous spines, 

 as they still are in our present Sea-urchins known as Cidaris ; 

 these areas were sometimes almost linear and sinuous. The 

 inter-ambulacral plates were numerous and arranged in mosaic, 1 

 instead of merely forming two alternate rows in each radial 

 area. Nevertheless true Cidaris appear in the Carboniferous 

 and even Diademse with their long spikes, hollow and fragile, 

 similar to those of the Mediterranean species, which have 

 their ambulacral areas already broadened, and bearing 

 ambulacral pores arranged in groups of three pairs. The 

 Crinoids have comparatively short arms ; their calyx comprises 

 three rings of plates ; five radial, bearing the arms ; five 

 basal, placed below and alternating with the latter ; and 

 sab-basal plates, occasionally but three in number, con- 

 necting the ring of basals with the stem. The last of these 

 rings of plates and often the penultimate ring are absent in 

 existing Crinoids. Such are the salient characters of the early 

 Echinoiderm fauna. 



We sketched on page 135 the probable descent of the 

 Molluscs. The Gasteropods and the Cephalopods may have 

 evolved simultaneously. The first preserved a broad ventral 

 sole, probably provided with lateral lobes utilized for swimming ; 

 in the second the ventral sole diminished until it was reduced 

 to the region around the mouth. Both must originally have 

 had straight shells, which later on developed a spiral form 

 in the swimming species and a corkscrew form among the 

 Gasteropods which had reverted to the crawling habit. 

 Palaeontology confirms these deductions, which in turn throw 

 light on certain palaeontological problems. If the Gasteropods 

 were really descended from the Chitons, and if their shell 

 was derived from the dorsal plates of the latter, the shell of 

 the oldest forms should be formed of triangular plates 

 juxtaposed from top to bottom. The shell of the Comdaria 

 fulfils this condition. That of the subsequent forms ought 



1 Melonites, Lepidocentrus, Cystocidaris, Bolhriocidaris. 



