xliv 
SYNOPSIS OF GENERA. 
genus erected by various authors have not been founded on the broadest 
conception of the value of slight variations. 
With the single exception (P. longicaudus), all the American Devonian species 
possess ten thoracic segments; this one has but nine, and were this feature to 
be accorded generic significance, the species would fall under the subdivision 
Xiphngonium, Corda (Prodrom Monogr. bbhm. Trilob., 1847). Ten however is 
the normal number of segments for the genus, and the possession of but nine 
(P. sculptus, Barrande), or even of eight (P. Barrandii, P. A. Roemer), is perhaps 
to be 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 Proldus 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¬ 
acteristic glabellar furrows. In the former condition of preservation P. fol¬ 
liceps, P. 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 Monia, Burmeister. In other states of preservation 
P. folliceps, P. Haldemani, P. Rowi and P. macrocephalus would fall under the 
recently established genus Dechenella, Kayser. Therefore, and for the addi¬ 
tional reason that even upon the internal surface of the glabella the furrows 
are subject to great variation in development (in P. crassimarginatus and 
