ERONTEUS. 37 



flabellifer by having the ribs broader than the furrows and by being less circular. 

 The reason I have concluded the heads above described to belong to the present 

 species is the similarity in their ornamentation ; and on this supposition the head 

 of B. pardalios differs from that of its congeners by its coarse granulation, by the 

 faintness of the depressions on its glabella, by the regular slant of its profile, and 

 by the angle of the sides of the glabella. 



From the extreme tenuity of the test these fossils are peculiarly liable to 

 distortion. Possibly it was not only thin but somewhat elastic. The heads 

 rarely occur perfect. As a rule the central portion, defined by the facial 

 sutures, is the only part present ; more rarely the free cheek bearing the eye is 

 preserved. This eye is a most beautiful object both from its symmetrical shape, 

 and from the large number and graceful arrangement of its lenses. These are so 

 placed as to present arching rows when viewed from two, or in some parts even 

 from three, directions. In one specimen, although not completely cleared of matrix, 

 fifty-six rows were counted, and in one of the central rows no less than thirty-six 

 facets. It is therefore probable that the total number of lenses did not fall far 

 short of 1800 or 2000. From the convex shape of this eye the animal must have 

 had the power of seeing upwards as well as, like Pliacops, laterally ; and this would 

 be further aided by the elevation of the cheeks on which it is placed. It is interesting 

 to observe that, while in general the heads of Trilobites occur much more 

 frequently at Lummaton than do the tails, the reverse is the case in the present 

 genus. The explanation of this seems to lie in the construction of these tails, 

 the radiating furrows of which must have added considerably to their strength. 

 These furrows are to be seen as strong ridges upon the under side. The test 

 appears to have been continued over a large portion of the under surface of the 

 tail so as to form a kind of sack or pouch. In a specimen in my collection this 

 part is seen to consist of two or three extremely thin layers which are not 

 indented by the furrows of the upper test. Of these the external layer appears to 

 have been granular, and the inner surface of the innermost is marked by very fine 

 and slightly waving concentric threads. The latter are often to be observed in 

 specimens where portions of the upper test have been removed. 



There is a curious case of malformation in a large tail of this species in the 

 Lee Collection in the British Museum. The furrows instead of radiating regularly 

 from the axis to the border are quite irregular, sometimes looped together, and 

 sometimes vanishing midway so as to leave broad marginal expanses only marked 

 with tubercles. A similar malformation, though of a very much less extent, is 

 seen in one of the specimens figured (Plate III, fig. 8). 



With the exception of the species already mentioned I know of few that could 

 be mistaken for the present form. Most of the Bohemian forms are easily 

 distinguished either by the bifurcation of the middle rib of the tail or by the orna- 



