405 



Length of head In largest specimen seen four Hnes ; of glabella about 

 three lines ; -width of glabella at base two lines, a little narrower in front 

 Surface with the exception of the striated and punctured front, apparently 

 smooth. 



I'mestone, No. 1. 



Fig. 384. 



Fig. 384.— The pygidium represented by Fig. 384, appears to belong to a, species of 

 Dikelocep/talus, but Ibe small fragment of stone in which it occurs resembles 

 Limestone No. 2, in which no recognisable fragments of that genua have been found. 



Genera. — Arionkllus (Barrande) and Menocephalus (Owen). 



These two genera seem to be closely related, and I shall therefore notice 

 them collectively. In Arionellus the glabella is cyhndrical or sub-conical 

 with three or four lateral furrows. The facial suture proceeds from the 

 eye forward with a slight inward inclination to the front margin which it 

 cuts on a line drawn between the eye and the glabella parallel with the 

 axis of the body. Behind the eye it cuts the posterior margin at a point 

 situated on a line drawn between that organ and the outer angle of the 

 head. In the thorax of A. ceticejjhalus there are from 7 to 16 segments 

 according to the age of the individual. The pygidium is small. 



The head (all that is known) of Menocephalus only differs from Ario- 

 nellus in having the glabella exceedingly convex. Owen discovered the 

 glabella and a portion of the cheek plate, but none of the other parts. He 

 describes the former as being circular, highly arched, hemispherical and 

 pustulated. Judging from this description and the figure given by the 

 author, and also from the aspect of the associated fossils, it appears to me 

 highly probable that the species which I have called M. globosus is not 

 'Only congeneric with Owen's M. Minnesotensis but that it is closely allied 

 thereto. M. Sedgewicki cannot be separated genericaUy from M. glohosus, 

 and this latter leads us through A. subolavatus to A. cylindricus. 



This latter appears to me to be an Arionellus. The specimens are too 

 imperfect to enable me to prove whether or not our four species belong to 

 two distinct genera or one only. I shall place the two most convex forms 

 in Menocephalus, and the other two in Arionellus. 



