88 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



genera and species spring into existence ; while in the Silurian this 

 division reaches its maximum developement ; in the Trias, the 

 straight Orthoceras forms become extinct, while the coiled forms 

 continue in decreasing numbers through the succeeding strata to 

 the present day. 



The Ammonoidea are first found in strata which Barrande 

 correlated as Silurian, but Kayser has recently made these beds 

 equivalent to the Hercynian, or lower Devonian. 



The early forms belong entirely to the Goniatites ; in the upper 

 Devonian the puzzling Clymeniadse appear, only to die out soon 

 after. In the Permian beds of the Artinsk region in the Urals/ 6 ) 

 the Salt Range of India,(7) and Texas, Nebraska, &c, the last of the 

 Goniatites and the first of the true Ammonites appear. In the 

 Trias the latter become important constituents of the world's fauna ; 

 and by their apparent sensitive natures, unable to withstand alter- 

 ations in their surroundings, form not only important zone fossils, 

 but have even been used as tests of Climate^) : thus we get in the 

 Jurassic rocks on either side of the equator, the delicate Phylloceras 

 and Lytoceras in great abundance ; in the northern latitudes, how- 

 ever, these two genera are never found, while sturdy forms like 

 Cardioceras, take their place. In the Cretaceous, many of the 

 Ammonites become curiously modified as if trying to withstand the 

 new order of things by taking fantastic shapes, such as the hooked 

 Hamites, the straight Baculites and Ptychites, the loose spirals like 

 Pictetia and Grioceras, &c, while some of them returned to the 

 ancestral Triassic types, such as Buchiceras. A similar story occurs 

 in the Juvavian province of the Trias, whose beds were probably laid 

 down in a sea gradually drying up ; here there occur the aberrant 

 Choristoceras, Bhabdoceras, and Cochlioceras, paralleling respec- 

 tively Crioceras, Baculites, and Turrilites of the Chalk. In the 

 uppermost Chalk (Danian), the Ammonites cease, though an Am- 

 monite and a Baculite from Chico Creek in California have been 

 referred to the Eocene. ( 9 ) 



The Dibranchiata first appear in the rocks as the Belemnoidea, 

 recently traced as descending from Orthoceras through the Triassic 

 AulacocerasS 10 ) These consist of a " phragmacone " similarly 



(6). Karpinsky, Sur 1'Amni. Fauna Artinski, Melanges geol. et paleont. tires du 

 Bull, de VAcad. des Sci., St. Petersburg, Vol. I, 1890. 



(7). Waagen, Salt Range, Fossils. Mem. Geol. Surv. India, 1879. 



(8). Neurnayr, Ueber Cliraat. Zonen der Jura und Kreide, Denkscrift Akad. 

 Wien, Vol. XLVII, 1883. 



(9). Trask, An Amnion, and Baculite from Chico Creek, California, Acad. Nat. 

 Sci. Proc, Vol I, p. 92, 1856 ; also Heilprin, Amnion, in Tertiary Rocks, Philadelp. 

 Acad. Nat. Sci. Proc, Vol. 34, p. 94, 1882. 



(10). Mojsisovics, Mediterran. Trias Prov., Abh. der K. E. Geol. Beichs, Wien 

 Bd. X, 1882. 



