MOLLUSCA — CEPHALOPODA. 



167 



Trachyceras Aon (Fig. 95 a), marking a well-known horizon Gallery 

 rich in fossils. Ceratites with its many rounded saddles, ^II- 

 recalling Prolecanites, is also characteristic of the Trias 

 (Fig. 94) ; a section of C. nodosus shows how the septa are 

 at first concave as in older forms and then become convex 

 towards the shell-aperture, as in ammonites generally. In 

 Arcestes intuslabiatus the periodical constrictions of the 

 coiled cone provoke enquiry as to their cause. 



Passing to the Lias, one notes many type-specimens of Table-cases 

 species founded by the Sowerbys and by T. Wright, as well -v^fx^cases 

 as examples of the ammonites that give their names to the 12 & 11. 

 successive horizons or zones into which the Lias has been 

 divided. Of these the oldest is Psiloceras jJ/(X?^07^??^s, the 

 earliest English ammonite; beginning with slightly ribbed 

 whorls, it reverts in the adult to the smooth shell of the 

 older types. A fine slab covered with iridescent shells of Between 

 this ammonite is placed on the wall. Among the British ^I'^'^^il^^ 

 specimens from the Lower Lias, an example of Acgoccras 

 heterogenes [C 1870] shows ribbed inner whorls like tliose Table-case 

 of the adjoining A. capricornus, and outer whorls with 6. 

 tubercles, as in Liparoceras striatum. Such changes from 

 smooth to ribbed, from ribbed to tuberculate, characterise 

 many ascending lines of ammonite evolution. Here are 

 to be seen shells of AmUycoccras j9/(X7^^cos^a, of which a 

 thick bed was formerly worked as an ornamental marble at 

 Marston near Yeovil. Slabs of this showing weathered and Between 

 polished surfaces are placed on the adjoining wall (Plate VII). 

 The larger specimens from the Lower Lias include a very Wall-cas'e 

 fine example of Coroniceras BucUaiidi showing the coronet 12. 

 of blunt spines from which the genus takes its name. 

 N"ear this is a large Asteroceras stellare from Lyme Eegis, 

 cut in half and showinsj the chambers dislocated durim^ 

 fossilisation. Asteroceras ohtusum shows the keel and the 

 simple suture contrasting with the rather complex one of 

 Coroniceras. Here is a Deroceras armatum with its big 

 spines. Above are some large specimens of the rare 

 Vermicerccs Conybearei. Between the cases is the largest 

 known Lias ammonite, about 1 metre (3 feet 4 inches) in 

 diameter, possibly an old individual of the last species. 

 Among species from the Middle Lias a noteworthy one is Table-case 

 Lytoceras fimbriatum, with sharp ridges at intervals indicating -vvall-case 

 that from time to time the aperture of the shell flared hb. 

 outwards, for reasons at which we can only guess (Fig. 95 c) ; 

 these flares cut across the ordinary fine ribs of the shell ; in 



