365 



Gasteropoda of the holostomata or vegetable feeders ; they are 

 stationed on their natural feeding- ground, the great sea tangles 

 and other broad-leaved plants of the Laminariacece. These 

 shells are further described under the section Gasteropoda. 

 Altogether, the proofs from petrology and palseontology accord, 

 to clearly make out the nature and position of the Churchdown 

 beds, namely, the sands of the Spinatus Zone. 



(2.) Special Paleontology (MoUuscan). — (a) Cephalopoda. 

 Nautili are rare in these sands; the specimens we possess are the 

 Nautilus aratus Dumoetier, and the N. intermedius of Sowreby : 

 on the contrary, the Ammonites of the zone are fairly distributed 

 as to number, though of comparatively few species. The shells 

 of the radians group now begin to come in as it were, for the 

 first time, indicating, most probably, a break in the deposition 

 of the beds. Both Amaltheus spinatus, and Harpoceras 

 Normanianum, occur often in the form of casts, the shell 

 having been dissolved away since the upheaval of the beds, 

 neither A. margaritatus nor its varieties have been secured ; 

 though had it been found, its significancy would be little, 

 as the geologist does not expect to meet in nature with 

 the sharp lines and rigid treatment that he does in books. The 

 new forms that now begin to present themselves, belong to 

 the Falciferi, and of the type radians. Of the specimens that 

 must be referred to this class of the Ammonitidce — first there is 

 Harpoceras Normanianum, which through its peculiarity of ser- 

 pentine ribbing, was originally named "radians amalthei." Other 

 species found in the Spinatus Zone are A. Zetes, and A. 

 Thouarsensis of A. d'ORBiGNY, as well as A. Mgion, or A. Acta;on 

 of the same author ; all of these partake of the intermediate 

 form between Phylloceras heterophylium and A. radians.* The 

 specimens are frequently imperfect, and many of them young. 

 But a good example illustrating the intermediate degree at this 

 stage is to be se^ in the collection of the Gloucester Museum, 

 labelled A. amalthei spinatus, Quenstedt. The difficulty felt in 

 exactly defining this border class of delicate gradation of character 



*Ammmitr.s Aalc7um-&gB,in, is the link between Am. Radians and Am. 



Opalinus of the Lower Oolites. 



