﻿24 EVOLUTION IN THE PAST 



this modification brought great rewards in the lampshell 

 competitions for survival. 



BIVALVED Bivalved molluscs do not seem to have greatly increased : 



MOLLUSCS nor were they very widely distributed. There was, however, 

 a little more variety in their ranks, for some were now in view 

 with flaps to their shells (Pterineidce), as with the wing-shells 

 of our own time (Avicula). This development, no doubt, 

 provided the hinge with an extra strong basis ; and the hinge 

 question was as important in molluscan as in lampshell life. 

 Many forms were doubtless still living in old-fashioned shells 

 without hinges ; and in the case of some others the hinge was 

 only imperfectly developed. But a few forms — forerunners 

 of which had appeared in the Cambrian — had shell-hinges 

 which were of superior mechanism to those of the most 

 progressive lampshells (Ctenodonta). Such animals, therefore, 

 were well equipped against burglarious starfishes, and other 

 shell-intruders in the shallows. 

 UNIVALVED Winged snails had less weight to carry than of old, for 

 MOLLUSCS their shells were now of much thinner texture (Cornularia). 

 New species of crawling snails were also in the seas ; but 

 there is no reason to suppose that they had undergone any 

 very important modifications of internal anatomy. Some 

 of them now possessed on the upper part of the foot a hard 

 growth (operculum), which served as a trap-door to the shell 

 as the animal lay coiled up within (Maclurea). Hospitality 

 was more rigorously denied. The more expansive forms with 

 well-raised towers to their shells (Cyclonema) were notably 

 on the increase. Others (Helicotoma), to judge by their shells, 

 had had their flanks considerably flattened, and had probably 

 become more athletic. 



SCAPHOPODS Scaphopods or " tooth-shells " were in existence at this 

 time (Dentaliurn) — spending their lives more or less embedded 

 in the mud and sand. They added nothing, therefore, to the 

 gaiety — little enough — of marine life. 

 chitons Other animals — equally stolid — were ancestors of the 

 " armour-shells " or chitons of our own time ; and, save for 

 their shell-plates not being so firmly secured to the margin 

 of the mantle, they seem to have differed but little from the 

 forms now living (Priscochitori). 



