CRETACEOUS PERIOD 109 



demands of Evolution, and were struck off the molluscan 

 roll. 



The most remarkable feature in the Cretaceous annals LAMPSHELLS 

 of brachiopods, or lampshells, was the great prosperity 

 attending an old family, which had in Ordovician times 

 dissented from other brachiopods. The dissent consisted in 

 discarding the stalk, and developing a cement as means of 

 fixation (Craniidaf). This persevering nonconformist family 

 is still living. Two other old families notable for beaked 

 shells that had taken a new lease of life in the Trias, 

 were also much to the fore in the Cretaceous Period (Tere- 

 bratulidcz, Rhynchonellidce). 



Among polyzoans, the older-developed colonies, with POLYZOANS 

 zooids in open-mouthed tubes (Cyclostomata), were for a 

 great part of the Period keeping well ahead of the " lid- 

 mouthed " colonies (Cheilostomata). In late Cretaceous 

 times, the latter seem to have drawn abreast of their rivals. 

 The reformed colonies, flushed with success, were in some 

 cases effecting changes in their social system. Certain zooids 

 were losing their status by being modified into organs 

 (avicularia) for keeping off intruders, and for seizing food 

 for their fellow-colonists (Cellaria, Onycho cello}. Zooids in 

 close proximity to these transformed individuals must have 

 lived in great ease and luxury. A privileged class, in short, 

 had risen in the community. Another change was in pro- 

 gress in some colonies. Certain zooids were being transformed 

 into menial appendages (vibracula*) for bringing a colony to 

 anchor (Lunulites}. 



Sea-urchins continued dominant among the spiny-skinned SEA-URCHINS 

 creatures, and underwent great developments as the Period 

 advanced. Old forms of more or less sedentary habits, with 

 circular tests, were very numerous ; and new forms of these 

 were making their appearance (Salenia). But it was among 

 the more inquisitive urchins, that had, by burrowing or more 

 active locomotion, lost their circularity, that a notable 

 increase was taking place. Heart-shaped forms sometimes 

 termed heart-urchins were very much in evidence (Holaster, 

 Micraster, etc.) ; and some were becoming still more like 

 ordinary bilateral animals, as the mouth was well in process 



