xliv SYNOPSIS OF GENERA. 



lus erected by various authors have not been founded on the broadest 

 conception of the value of slight variations. 



\\ ith the single exception (P. longicaudus),n\\ the American Devonian species 

 possess ten thoracic segments; this one has but nine, and were this feature to 

 he accorded generic significance, the species would fall under the subdivision 

 Xiphogonium, Corda (Prodrom Monogr. bohm. Trilob., 1847). Ten however is 

 the normal number of segments for the genus, and the possession of but nine 

 I /'. Kulptus, Barrande), or even of eight (P. Barrandii, P. A. Roemer), is perhaps 

 to I"' regarded as an embryological character retained in maturity. It has been 

 customary to constitute subdivisions of this genus, and even distinct genera, 

 upon the variations in the form of the glabella, and in the apparent number 

 and length of the transverse glabellar furrows. Evidently the fact has fre- 

 quently been overlooked that a glabella which evinces no trace of these fur- 

 rows upon the external surface, will, under favorable weathering or upon a cast 

 of the internal surface, show the normal number of four pairs and the poste- 

 rior accessory pair of furrows. In occasional instances, where the ornamenta- 

 tion of the crust of the glabella has been preserved without defect, traces of 

 all these furrows may be observed, but this is not true of species in which the 

 glabella is normally smooth. The vertical compression of the crust, causing 

 it to give way along the furrows, often develops them, or the slight detach- 

 ment of the crust from its matrix makes them apparent by translucence. 

 Thus the species Prottus folliceps, P. crassimarginatus, P. Haldemani, P. Rowi, 

 in normal preservation, have a smooth, unfurrowed dorsal surface, but either 

 in casts of the internal surface, or in cases of compression, show all the char- 

 eristic glabellar furrows. In the former condition of preservation P. fol- 

 liceps, I', crassimarginatus, P. clarus, P. Haldemani, P. Rowi and P. macrocephalus 

 would follow the type of Gerastos, Goldfuss; P. canaliculatus and P . longicaudus 

 would belong to the genus Mania, Burmeister. In other states of preservation 

 P. folliceps, P. Haldemani, P. Rowi and P. macrocephalus would fall under the 

 recentl) established genus Dechenella, Kayser. Therefore, and for the addi- 

 tional reason tli.it even upon the internal surface of the glabella the furrows 

 are subject to great variation in development (in P. crassimarginatus and 



