THE CRETACEOUS PERIOD. 189 



the ponderous Gryphcea vesicularis and Exogyra ponderosa, with Ostrea larva in 

 great abundance, are conspicuous fauna! elements. In the Wenonah beds, 

 the uppermost member of the group, there is a return of many of the species 

 of the earliest subfauna, implying that the fluctuations in life were local. 



A more marked faunal change then ensued, corresponding approximately 

 to the transition from the Matawan to the Monmouth, in which a new immi- 

 grant element is introduced, characterized by Belemnitella americana and 

 Terebratella plicata. There is at the same time a recurrence of the big oysters, 

 Gryphcea vesicularis, Exogyra costata, and Ostrcea larva. Within the Monmouth 

 formation there are also recurrences of other Matawan species. The above, in a 

 general way, stand for the faunas of the lower portion of the series, south to 

 the Mississippi embayment, including the Eutaw and Ripley faunal groups. 



At the north, the Rancocas fauna was characterized by the brachiopod 

 Terebratula harlani, associated with many Gryphcea vesicularis that lived on from 

 the earlier stages, and, especially in the Vincentown lime sand, by the great 

 numbers of bryozoans and shells of foraminifera. The uppermost horizon, the 

 Manasquan, is characterized by Caryatis veta and Crassatelladela icaroisis. 



In the Texan province, the lower divisions contain many species common 

 to the faunas of the Atlantic coast, implying close relations. The recurrences 

 of the species above noted are probably but expressions of migrations to and 

 fro in the Atlantic-Gulf coastal tract, as the local conditions varied. The most 

 marked departure from the Atlantic faunas was in the chalk formation (Austin 

 limestone), in which the foraminifers Textularia and Globigerina, and the sea 

 urchins Hemiaster and Cassidulus, were important features. The Inoccrauius 

 and the ammonites also played a much more conspicuous part, and the fauna 

 was otherwise related to that of the great interior sea. 



Explanation of Fig. 417. — Cephalopoda, a, Nautilus meekanus Whitf., one of the 

 simplest types of closely coiled cephalopods. Note the smooth shell and the simple 

 sutures; b, c, Prionotropis woolgari (Mantell), a normal ammonite, with highly 

 ornamented shell and moderately complex sutures; d, Scaphites nodosus Owen, an 

 ammonite exhibiting a slight tendency to uncoil in the last volution; e, /, Helicoceras 

 stcvensoni Whitf., an ammonite coiled in a heliciform spiral, with its highly compli- 

 cated suture; g, Bacidites grandis M. and H., a straightened-out ammonite, with 

 a moderately complex suture. In its infantile stage, this form starts as a closely 

 coiled shell; h, i, Ptychoceras crassum Whitf., an ammonite which, in the stage 

 shown in the figure, is no longer coiled but recurves upon itself. Pelecypoda: 

 j, Inoceramus vanuxemi M. and H., a representative of one of the most character- 

 istic genera of Cretaceous shells; k, Ostrea solcniscus Meek, a representative of a 

 genus which with its near allies reached its greatest development in the Creta- 

 ceous period; I, Idoncarca nebrascensis Owen, a shell allied to the areas of the 

 recent seas. Gastropoda: to, Pyropsis bairdi (M. and EL), n, Drepanochilus 

 nebrascensis (E. and S.), o, Aphorrhais prolabiata (White), p, Neptunella inter- 

 textus (M. and H.). The canaliculate and modified apertures of these shells 

 differentiate them sharply from the ancient Paleozoic types of gastropods, and sug- 

 gest some of the shells of recent seas (Compare with Tertiary Figs.). Echinoidea: 

 q, r,Salenia tumidida Clark; s, Pedinopsis pondi Clark, two forms of regular sea 

 urchins in which the only lack of radial symmetry is in the apical system of plates, 

 as is well shown in q; t, Botriopygus alabamensis Clark; u, Cassidulus subquadratus 

 Con., two sea-urchins in which the bilateral symmetry is strongly developed. 

 (Weller.) 



