16 LIAS OF ENGLAND AND WALES: 
characterize different horizons. Some Palaeozoic forms like Spiri- 
ferina and Lcptana have lingered on to Liassic times, while some 
new genera like Thecidium make their appearance. Most 
abundant are species of Rliynchonella, Tercbratula, and Wald- 
heimia. Polyzoa are found abundantly in some of the calcareous 
strata. 
Crustacea are well represented, although the larger forms are not 
particularly abundant. The chief groups had appeared previous 
to Jurassic times. The Macroura include Eryon and Glyphca. 
Isopods are occasionally preserved, while Ostracods have been 
found in abundance at certain horizons. 
Of the Insects, though the remains that have been obtained 
indicate a great number and variety of forms, they are mostly frag- 
mentary, consisting of wings and elytra. Annelides, represented 
by Serpula, Ditrupa, and other genera are not uncommon. 
Echinodermata are plentifully preserved in the limestones, and 
more especially in the oolitic beds. The Crinoids include 
Pentacrinus, Extracrinus, and Apiocrinus. The Star-fishes are 
represented by Opldodcrma, found in calcareo-arenaceous strata. 
The Echinoids include Acrosalcnia, Cidaris, Ilemicidaris, Echino- 
brissus, Clypcus, and many other genera. 
Corals are plentiful at particular horizons, in the calcareous 
strata. In the Lias they occur abundantly at* but few localities. 
In the Oolites we find occasional Coral-banks of somewhat 
limited extent, but little or n:> evidence of particular reefs. The 
more abundant genera include Eunomia, Isastreea, Montlivaltia, 
Slylina, Thamnastrcea, arid Thccosmilia. 
Hydrozoa are represented by a form allied to Solcnopora. 
Sponges are rare; a few marine and one freshwater species have 
been recorded. Numerous Foraminifera have been obtained, 
mostly from particular beds and localities. 
Preservation of Fossils. 
While organic remains are as a rule more varied and abundant 
in the limestones, the fossils are not always so well preserved as in 
the clays. In the limestones the shells of the Mollusca, and 
frequently the Corals, may be replaced by structureless calcite. 
Occasionally as in the Portland " Roach," the shells have been 
entirely removed, and we have moulds and interior casts remaining. 
In the cla}S ihe fossils are usually well preserved, and sometimes 
when the outer layers of Molluscan shells have disappeared they 
exhibit their inner nacreous layers. Such iridescent fossils aj'e 
to been seen in theLias of Watchet and other places, and in the 
Kimeridge Clay of Brill arid Market Rasen. 
In the sandy strata we find comparatively few fossils ; and as 
noticed by Mr. Clemen I Reid,* loamy sands appear to be very poor 
preservers of fossils. This is notably the case with the micaceous 
sandy loams of the Middle Lias. The fossils in many cases 
* Pliocene Deposits of Britain, p. 132. 
