MOLLUSCA — CEPHALOPODA. 
167 
of J.'mc/i.i/ccms yion (Fig. marking a well-known horizon 
rich in fossils. Cemfites with its many rounded saddles, 
recalling Frolecanitcs, 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 
Arccstes intuslahiatus the periodical constrictions of the 
coiled cone provoke enquiry as to their cause. 
Passing to the Lias, one notes many type-specimens of 
species founded by the Sowerbys and by T. Wright, as well 
as examples of the ammonites that give their names to the 
successive horizons or zones into which the Lias has been 
divided. Of these the oldest is Psiloccras planorhis, the 
earliest l^nglish ammonite; beginning with slightly ribbed 
wliorls, it reverts in the adult to the smooth shell of the 
older types. A fine slab covered with iridescent shells of 
this ammonite is placed on the wall. Among the British 
specimens from the Lower Lias, an example of Liparoccras 
hetcrorjcnes [C 1870] shows ribbed inner whorls like those 
of the adjoining L. capricormi, and outer whorls with 
tubercles, as in Liparoccras striatum. Such changes from 
smooth to ribbed, from ribbed to tuberculate, characterise 
many ascending lines of ammonite evolution. Llero are 
to be seen' shells of Amhlycoccms plaaicosta, of which a 
thick bed was formerly worked as an ornamental marble at 
Marstun near Yeovil. Slabs of this, showing weathered and 
polished surfaces, are placed on the adjoining wall (Plate Vll). 
The larger specimens from the Lower Lias include a very 
fine example of Coroiiiccras Bucldandi showing the coronet 
of blunt spines from which the genus takes its name. 
Xear this is a large Astcroceras stellare from Lyme Eegis, 
cut in half and showing the chambers dislocated during 
fossilisation. Astcroceras ohtusum shows the keel and the 
simple suture contrasting with the rather complex one of 
Coroniceras. Here is a Deroccras arniatum with its big 
spines. Above are some large specimens of the rare 
Vermiceras Conyhearei. Betw'een 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 
Lytoccras Jimbriatum, wdth sharp ridges at intervals indicating 
that from time to time the aperture of the shell flared 
outwards, for reasons at which we can only guess (Fig. 95 c) ; 
these flares cut across the ordinary fine ribs of the sliell ; in 
Gallery 
VII. 
Table-eases 
6 & 7 . 
Wall-cases 
12 & 11 . 
Between 
W all-cases 
11 & 12 . 
Table-case 
6 . 
Between 
Wall-cases 
12 & 13 . 
Wall-case 
12 . 
Table-case 
7 . 
W all-case 
llB. 
