JURASSIC PERIOD 85 



generically identical with that of their Uving brethren 

 {Spondylus). Other forms appear to have been what are now 

 known as " edible " oysters. They were probably relished, 

 at this time, mainly by whelks and starfishes. 



Pearl-oysters were here, with their beady products {Melea- 

 grina). Clams also were to be seen ; but they did not much 

 resemble modern forms (Dicer as). File-shells, of which 

 forerunners had appeared as early as the Carboniferous 

 Period, were now abroad of much larger size (Plagiostomd). 

 " Ship- worms " — molluscs so-called from their affection for 

 boring into floating timber — had also appeared (Teredo) ; 

 and doubtless gratified their tastes on flotsam tree-trunks 

 from the forests. Other bivalves were forerunners of Venus- 

 shells, and tellens ; and if their shelly coverings were as 

 gaily painted as those of their living descendants, their 

 collecting-grounds must have mirrored as flower-beds sub- 

 merged (Venus, Tellina). 



Gastropods still differed in many respects from living univalved 

 forms. It is interesting to note that those provided with molluscs 

 a siphon apparatus had clearly derived health and strength 

 from the contrivance. Such marine snails were vastly more 

 numerous than in the Trias ; and among them were forms 

 recognisable as cowries, pelican-foot, and scorpion shells 

 {Cyprcea, Aforrhais, Harfagodes). 



Ammonites continued predominant among cephalopods, ammonites 

 and indeed among all marine invertebrates. Some Triassic 

 families were still represented ; but most of them had either 

 been suppressed or transformed. Evidence of breakdown, 

 visible in a shght degree at the close of the Trias, was now 

 more marked. The adaptive powers, however, of the animals 

 were still far from being exhausted ; as evidenced by the 

 various modifications to meet increasing difficulties. 



Most of the Jurassic forms possessed shells well strength- 

 ened with ribs. These bands of support were of various 

 designs, suggestive of rings (Lytoceras), straps (Aspidoceras), 

 loops (Stephanoceras), and wavelets (Mgoceras). In not a 

 few cases the shell was further strengthened by the presence 

 of a keel, which must also have brought advantages in the 

 matter of stabihty {Cardioceras, Harpoceras, etc.). 



