ORDER III PROPARIA 633 
Family 7. Acidaspidae. Barrande. 
Dorsal shield spinose. Cephalon transversely semi-elliptical, quadrate, or trapezordal ; 
genal angles spiniform. Glabella with one large median axial lobe and two or three 
lateral lobes. Free cheeks large, separate. Sutures extending from just within the genal 
angles abruptly inward to the eyes, and then forward, 
cutting the anterior margin each side of the glabella. 
Eyes small, often prominent. Thorax of eight to ten 
segments, with ridged pleura extended into hollow spines. 
Pygidium usually small, with spinous margin. Ordo- =m 
vician to Devonian. 5 te 


In this family, as well as in the Lichadidae, is to 
be found the highest expression of differentiation and 
specialisation among the Opisthoparia. The primitive 
pentamerous lobation of the axis of the cranidium is 
entirely obscured, and is only clearly seen in the protaspis 
and early nepionic stages. These two families are very 
closely related, the chief differences being noted in the 
size and character of the pygidium, and the ribbed or 
grooved pleura. The Lichades are generally much 
larger and flatter, but the smaller and spinose forms of 
Arges and Ceratolichas approach quite near some of the 
Acidaspidae. 
Acidaspis, Murch. (Figs. 1276, 1288, J, 1316). 
Ordovician to Devonian ; Europe and North America. ejdasnis Dufrenoyi, Barr. Silurian 
Dicranurus, Conrad; Ancyropyge, Clarke; Devonian, (Et. E); St. Iwan, Bohemia (after 
; . jaan Barrande). 
Selenopeltis, Corda ; Ordovician. 
Fia. 1316. 
Order 3. PROPARIA. Beecher. 
Free cheeks not bearing the genal angles. Facial sutures extending from the lateral 
margins of the cephalon in front of the genal angles, inward and forward, cutting the 
anterior margin separately or uniting in front of the glabella. Compound paired eyes 
scarcely developed or sometimes absent in the most primitive family ; well developed and 
schizochroal in the highest family. 
This is the only order of Trilobites which apparently begins during the known 
Palaeozoic, and unlike the other orders, had no pre-Cambrian existence. The earliest 
forms of the Proparia were initiated at the close of the Cambrian and dawn of the 
Ordovician. The greatest generic differentiation of the group was early attained ; 
during the Silurian and Devonian a rapid decline ensued, and only one or two genera 
survived into the beginning of the Carboniferous. 
Among the Opisthoparia, it was shown that the Conocoryphidae formed the 
natural base or most primitive family in the order, and was distinguished by the 
narrow marginal free cheeks and absence of well-developed eyes. It is of great 
interest and importance to be able to recognise in the Proparia a similar primitive 
family having characters in common with the other, but still clearly belonging 
to the higher order. Placoparia, Areia, and Dindymene of the Encrinuridae 
constitute a group of apparently blind Trilobites with narrow marginal free cheeks, 
and present in general the appearance of Atops, Conocoryphe, Ctenocephalus, etc., of the 
Conocoryphidae. 
