42 TRENTON GROUP. 



acteristic species are Bythotrephis mcculens, Monticulipora lycoperdon, Schuoerimu 

 nodosus, Stictopora elegantxda, Orthis bellarugosa, 0. cequivalvis, Trochonema umbili- 

 catum, Subulites elongatus, and Helicotoma plamdata. 



§ 83. There are numerous species which continued to live until the Hudson 

 River age, and are therefore common to three Groups, as Strophomena abtemata, 

 S. rhomboidalis, Leptena sericea, Zygospira modesta, Rhynchonella capax, Calymene 

 callicephala , Asaphus gigas, and Ceraurus pleurexanUiemns. Such species are usually 

 quite variable in form and size, and seem to have changed to suit the conditions of 

 their habitat, and also, in accordance with the theory of evolution, to have reached 

 the climax of development, and subsequently gradually declined. Strophomena 

 rhomboidalis occurs in Trenton, Utica Slate, Hudson River, Clinton, Niagara, Lower 

 Helderberg, Upper Helderberg, Hamilton, Chemung, Waverly, Burlington, and 

 Keokuk Groups. Its vertical range exceeds that of any other species in any of the 

 rocks of the known world, and its geographical distribution is common to every con- 

 tinent where strata of these ages have been studied and described. The varietal forms 

 have been called S. tenxiistriata from the Lower Silurian, S. depressa from the Upper 

 Silurian, and S. rhomboidalis from the Devonian and Subcarboniferous. The Lower 

 Silurian specimens are usually smaller, and have fewer concentric wrinkles over 

 the visceral region, than those from the Upper Silurian and Devonian, while the 

 length of the front and lateral margins from the geniculation is usually greater in 

 the Upper Silurian than it is in the Lower Silurian, Devonian, or Subcarbon- 

 iferous specimens; but these differences are not so constant as to form inflexible 

 characters, and hence it is that many of the learned and better palaeontologists 

 have classed them all together under the first and oldest specific name. The vari- 

 ous forms which Strophomena alternata assume in the same Group of rocks are wonder- 

 ful ; the radiating striss differ in size and number ; the hinge line is sometimes longer 

 and at other times shorter than the greatest width of the shell. The shells are 

 sometimes much longer than wide, and at other times as much shorter. The 

 lateral sides are sometimes straight, and at other times rounded. Some shells are 

 nearly flat, others are deeply concave on the dorsal side and highly convex on 

 the ventral. Age in some specimens appears to have materially thickened the 

 shells, and preserved strong imbricating lines of growth, while in other cases we 

 have much larger shells that are very thin and destitute of imbrications. Like 

 differences may be distinguished in other species having great vertical distribution, 

 as in Rhynchonella capax and Zygospira modesta. 



§ 84. The rocks of this Group are composed almost entirely of remains of the 

 hard parts of animals that swarmed in the seas of that age. Some shells are pre- 

 served in good condition, but generally the comminuted fragments are held to- 

 gether by lime cement, forming the limestone strata, leaving well-preserved specimens 

 to be found only in the shaly partings. It is common to find that one animal has 

 grown upon another, as a IAchenocriniis upon a brachiopod, and a bryozoan upon 

 the former, under such circumstances as to show the shell was at the bottom of the 

 ocean during the growth of the Liehenoerinus, and that the latter must have ceased 

 to grow before the bryozoan attached. From this we infer the clearness of the 

 water, for otherwise mud would have intervened ; and we also infer a slow depo- 

 sition of materials, for the lives of two animals transpired before the deposit was 



