Raymond : Notes on Ordovician TRiLoniTES. 61 



('.ir\iii District. The whole struct iirc' of tin- free checks,- thorax, and 

 pygiciiiiin is very ciilTerent from tliat of Caphyra radians Barrande and 

 C. siriatiiliis W'alcott. 



Family ASAPHID^ Emmrich. 

 It is difficult to subdivide this family satisfactorily, but there are a few 

 facts that can not be overlooked in the classification of the genera. The 

 character of the hypostoma separates two great groups, one including 

 those forms with an entire hypostoma, while in the other grouj) the 

 hypostoma is forked. Geologically, the forms witli the entire hypostoma 

 are the more ancient, and this group includes the species in which the 

 lobation of the glabella and the segmentation of the pygidium are most 

 marked. It is in this group also that the anterior limb of the facial suture 

 is generally marginal, while in the group with forked hypostomas the 

 anterior limb of the suture is generally on the dorsal surface. In both 

 groups there is a tendency for the glabella to become flat and to merge 

 into the general contour of the cephalon. The glabellar furrows become 

 faint and finally obsolete, the axial lobe of the thorax becomes broader, 

 and the ribs on the pygidium tend to disappear, giving the whole animal 

 a smooth upper surface. This "smoothing out" reaches its culmination 

 in Nileits among the more primitive forms, and in Isotelus in the other 

 group. Of the first group onh- Vogdesia survives as late as the Black 

 River, the earlier forms appearing in the Cambrian. The earliest member 

 of the second group now known appears (in America) in the Upper Beek- 

 mantown, and the subfamily is very abundantly represented in the Chazy, 

 Black River, and Trenton, continuing throughout the Ordovician. 



Subfamily OGYGIN^ nov. 

 Asaphidce with hypostoma entire and eight segments in the thorax. 



Section l . Ogygince with anterior limb of facial suture marginal. Ogygia^ 

 Niobe, Asaphellus, S ymphysur lis, N ileus, Vogdesia, Illcenunis. 



Section 2. Ogygince with anterior limb of suture intra- marginal. Mega- 

 laspis, Mega'aspides. 



Subfamily ASAPHINjE nov. 

 Asaphidce with hypostoma forked. 



Section i. Asaphince with anterior limb of facial suture marginal. 

 Basilicus. 



'The Lower Paleozoic Trilobites of the Girvan District, Ayrshire, pt. I, 1903, 

 39. pi. 6. figs. 11-14. 



