86 EVOLUTION IN THE PAST 



Many ammonites, nearer the end of their resources, were 

 crawling about in shells with dwarfed body-chambers (Cosmo- 

 ceras, etc.) ; whilst some forms were beginning to uncoil, 

 seeking salvation, it would seem, by a reversion to the more 

 or less straight shell of far-back ancestors (Scaphites, Ancylo- 

 ceras). This retrogressive movement became a prominent 

 feature with ammonites, when their decline had fairly set in. 

 In spite, however, of their trials and adversities, ammonites 

 made a grand display in Jurassic seas. 



BELEMNITES Belemnites, meanwhile, although handicapped by being 

 the much-coveted food of many fishes and sea-reptiles, were 

 coming on with a flood tide. They continued for the most 

 part of small size ; but a few forms were now abroad fully a 

 yard in length (B. giganteus). Their ranks were certainly 

 presenting a less archaic character ; as some forms differed 

 but little from common squids or calamaries (Loligo vulgaris). 

 NAUTILUS The old straight-shelled nautiloids had by this time been 

 wellnigh weeded out : and the group, which comprised 

 them, was represented almost entirely by forms more or less 

 identical with the living nautilus. Nautiluses, although 

 developed on more lasting lines than Ammonites, were never- 

 theless quite unimportant cephalopods in Jurassic seas. 



Crustaceans, by readjustments in points of their structure 



that had been more or less vetoed by Nature, were moving 



SHRIMPS with the times. Well-developed shrimps and prawns were 



numerous (Penaus, Mger) ; and small creatures, fairly 



LOBSTERS describable as lobsters, were adding distinction to the crusted 



fraternity (Eryon, Mecochirus, etc.). Animals of the same 



family (Eryonidce) as some of these now wander in a sightless 



condition in abysmal ocean depths. 



CRABS Crabs, by a series of modifications from early shrimp life, 

 had now become distinguishable from their long-tailed fore- 

 runners. For the time being, however, they were insignificant, 

 alike in number and variety (Palceinachus, Prosopon). 

 BARNACLES Barnacles for the most part pursued an undemonstrative 

 career : but certain forms were setting their houses in order 

 by reducing their shell-plates, and arranging such as remained 

 in a more regular manner (Archceolepas). 



Up to this Period fishes had been rapidly increasing in 



