ai 
CROMUS INTERCOSTATUS, Barr, Sil. Syst. Bohéme, p. 824, Plate XLIII, 
figs. I-5. 
In which the head and tail is spined with 20-28 axial joints; pleurae 
12-16; all from Reg. E. See plate 2, fig. 22. 
All the species are characteristic of the typical Cryptonymus in hav- 
ing their genal angles of the head rounded off; pygidia with few joints, 
12-20; and few pleurae, 10-12-16, without the long caudal spine com- 
mon to those species which we have classed under Encrinurus. The 
replacement of fine granules, in place of coarse tubercles, gives a better 
view of the marking of the glabella, and exhibit 4 lobes on each side. 
The pygidia of C. Beaumonti and C. transiens are those of Cry ptony- 
mus, although the head of C. zatercostatus has the genal angles spined. 
It exhibits only the variation common to all the genera. 
The pygidia of C. ztercostatus and Lohemicus have the ends of the 
pleurae spinous with the Encrinuridez; they are notched at the end. 
There is a general tendency of all Trilobites with semi-cylindrical pleu- 
rae, to extend themselves into spinous terminations, and should only 
have specific value. 
Tornquist exhibits a species, &. schtsticola, from Dalecarlia, which 
has the terminal pleurae of the tail spinous. We also note that the 
tubercles on the axis of the tail vary in number from 1-5; in the centre 
the tendency is to become enlarged. ‘This characteristic is more in keep- 
ing with species of Cryptonymus than in Encrinurus, which has only the 
central row of nodes. 
In tracing these spinous forms from the Ordovician to the Silurian, 
we notice that each group begins its history in notched segmentation. 
This form of pleurae develops into spinous form; becomes more abundant 
at the culminating of the group, reached in Cromus intercostatus; after- 
wards the genus becomes extinct in the higher Silurian beds. 
ENCRINURUS Novakt, Frech, Zeitschr d. Deutsch Geol. Ges., 1887, 
Pp. 735, Plate X XIX, figs. 5-9 of Cromus Beaumonti. 
Cromus Murcursont, DeKoninck Rech. Foss. Pal. Sud. Aust., p. 55, 
Plate I, fig. 9. 
The species is distinguished from all others by the shape of the 
glabella and depth of the furrows which separate it from the fixed cheeks. 
