132 NEW YORK STATE MUSEUM 



a profusion of illustration as presented by the genus Dalmanites. Yet no 

 greater extreme is reached than in the species of Lichas we have here 

 described, Gasp e lichas forillonia, indeed the most extravagant 

 instance of the development of spines among the trilobites. The, early 

 Devonic forms of Homalonotus often become echinate and even the genus' 

 Phacops, the most compact and conservative expression of all the Pha- 

 copidae, which in boreal faunas of this time occasionally assumes spinules- 

 on' the genal angles and on the thorax, in austral faunas may put on a row 

 of spines about the margin of the head. (P. dagincourti Ulrich, 

 Bolivia.) 



In Dalmanites the modification of the head shield is distinctly earlier 

 than similar adornment of the tail. Cryphaeus and Coronura are forms 

 with pygidial modification in which the tail furnishes the generic characters 

 and these do not appertain to. the earliest Devonic faunas. No species of 

 theHelderberg-Oriskany or of those faunas we are here describing present 

 any of these integumental modifications upon the tail. The tail shield has 

 conserved' the typ€ better and. is the last to be affected. These changes 

 have not always progressed with equal foot. There are conservative species 

 of Dalmanites that continue -the generic type without. modification into the 

 early Devonic, the forms of Hausmannia or Odontochile, and it is these that 

 have prolonged the existence of the type. In the following series of dia- 

 grams we have given expression to these variations and their time relations. 



The variation in cephalic ornament indicated in. these sketches is. seldom 

 accompanied by corresponding variation in the tail. Almost without 

 exception the species whose cephala are here, figured have tails of the sirnple 

 triangular type, variation being manifest only in the degree of development 

 of the caudal spine. In two rather widely separated types of, frontal 

 development, D. longicaudatus and D. n.asutus, a very long cau- 

 dal spine is present ; Odontocephalus has a double tail spine (s e 1 en u r u s) 

 and Cryphaeus with a simply lobed head, as in figure i, has a fringe of five 

 flat leaf-shaped pleural lobes on each side and one axial., Otherwise th^ 

 pygidia are of practically the same expression and in, no; correspondence 

 with the variations of the cephalon. .... 



