MOLLUSCA — CEPHALOPODA. 



171 



H. varians, to the smooth H. Goupilianus. This again Gallery- 

 illustrates the change of ornament characteristic of a 

 declining series. Among tlie larger specimens the most ^^^'^^^^ 

 noteworthy are Fuzosia Austcni, Pacliydiscus j;gr«mj9/^6S, and ' 

 Pachydiscus leptopliyllus. Some specimens clearly show the 

 complex suture of this last, but the most interesting is the 

 very large one from Eottingdean, 3 feet 8 inches in diameter ; Between 

 the rapid increase in the width of the coiled cone may be 

 contrasted with the very slow increase in the large Lias 

 specimen opposite. The largest known ammonite is P. seppen- 

 radensis, from the Lower Senonian of Westphalia, with a 

 diameter of about 2 metres (6 ft. 8 in.). A plaster repro- 

 duction of it is fixed at the north end of the Gallery. 



We have already noticed the changes in ornament that 

 characterise ammonite races as they advance to and recede 

 from the acme of their development, and we have seen how 

 the suture likewise becomes more complex and then returns 

 to a simpler form. There is yet another and more obvious 

 change. Just as the ascending stocks, beginning with 

 straight forms, gradually coiled the shell more and more 

 closely, so, having reached their acme, they begin to uncoil 

 and may ultimately return to a straight condition, if they 

 do not previously become extinct. Further, instead of 

 merely unwinding, they may lose the regularity of the coil 

 and become wound in an asymmetrical spire or turret, like 

 that of most gastropod shells. Already in Triassic times the 

 Ceratites (using the term in a broad sense) show all these 

 retrogressive changes, ending in the straight Bhabdoceras. 

 Of the various ammonite families that passed into Jurassic 

 and Cretaceous times, the Stepheoceratidae gave off a 

 degenerate branch so early as the Bajocian Age. The 

 eccentric Gadomoceras has already been noticed, and here Table-case 

 are exhibited the further uncoiled Spiroceras [Crioceras] 8. 

 hifurcatmn and similar forms, which led to the straight 

 Bacidina of the Callovian. 



It was, however, chiefly towards the close of the Table-cases 

 Cretaceous Epoch that all the persisting races entered on -^^^^^^ 

 this degeneration. N'ames have been given to the various 3^ 

 stages of uncoiling, such as Crioceras, in which the whorls 

 are partly separate (Fig. 96 a); Macroscaphites with the last 

 whorl bent slightly back and then returning on itself 

 (Fig. 96 d); Sca2)hites with a somewhat closer coil to start 

 with and a more rapid return (Fig. 96 /) ; Hamites, which 

 starts with a small coil, then goes straight for some distance, 



