472 MESOZOIC TIME REPTILIAN AGE. 



Large stumps of Cycads have been found in Maryland near Baltimore; one is 

 12 inches in diameter and 15 high. (P. T. Tyson, who observes that they may 

 be Upper Jurassic.) 



2. Animals. 



Among Protozoans, the group of Rhizopods has a special im- 

 portance in the Cretaceous period. They are abundant in many 

 of the beds in New Jersey and other Cretaceous regions of North 

 America, though less so than in the chalk beds of Europe. In one 

 genus, Orbitolina, the species are disk-shaped, (fig. 750), and closely 

 resemble in form some of the Nummulites of the early Tertiary. 

 Sponges also are a common fossil, although little known thus far 

 in America. 



In the sub-kingdom of Mollusks, the more characteristic genera 

 of Conchifers are the three of Oysters, Ostrea (fig. 753), Gryphcea 

 (figs. 755, 756), and Exogyra (fig. 754) (species of which occurred 

 in the Jurassic period, but are more common and larger in the 

 Cretaceous), and Inoceramus (fig. 757), a genus related to Avicula 

 and Mytilus, some species of which are of great size and have the 

 surface in undulations. Exogyra and Inoceramus end with the Cre- 

 taceous, and but one Gryphcea is known afterward. 



Another group characteristic of the Chalk period, and, moreover, 

 not known after it, is that of the Rudistes (figs. 782-784). It includes 

 the genera Hippurites, Radiolites, Spherulites, and a few others. Hip- 

 purites has a long tapering form (fig. 782), somewhat like a nearly 

 straight, but rude, horn with a lid on the top, the lid being the 

 upper valve, and the conical portion the lower. Within there is 

 a subcylindrical, tapering cavity, having one or more projecting 

 ridges on the sides running the whole length. Fig. 782 a shows the 

 interior of one : there are two prominent ridges, but one is only 

 partly free in the interior space. The other genera have a similar 

 anomalous character, but differ in the interior. Fig. 783 repre- 

 sents the lid or upper valve of a Radiolites, showing the projections 

 below (b, c) to which the muscles closing the lid are attached; and 

 fig. 784 is the same in Spherulites. The Rudistes are supposed to be 

 related to Chama among the Dimyary Mollusks. 



Of Cephalopods, there are in the Cretaceous beds numerous Am- 

 monites and Belemnites. Some of the Ammonites from beyond the 

 Mississippi are over three feet in diameter. There is also a multi- 

 plication of other genera of the Ammonite family, the shells of 

 which are like Ammonites more or less uncoiled ; as Scaphites (figs. 

 706, 767), from scapha, a boat; Crioceras (fig. 786), from npioc, a ram's 

 horn; Ancyloceras (fig. 787), from aynuAri, a hook or handle; JIamites, 



