AGNOSTUS INCERTUS. 29 



to represent the second segment; terminates in a blunt point. Lateral lobes 

 considerably narrowed behind the axis, where they are confluent with one another. 

 Head- and tail-shields (judging from a distorted specimen) about 4 — 4*5 mm. 

 long and wide. 



Since the above description was printed I have found a less distorted 

 specimen, from Dwrrhyd, near Solva, in the Oxford University Museum. It 

 shows that the second segment of the axis of the tail was perfectly defined 

 posteriorly by a furrow in the position of the crack which crosses the axis in the 

 specimen represented in Plate II, fig. 24. The head- and tail-shields are about 

 4 mm. long and 5*5 mm. wide. 



Our specimens are slightly smaller than those described by Brogger, and the 

 head is somewhat less quadrate than in his figures. Matthew makes the same 

 remarks about the American form, which he refers to Agnostus parvifrons, var. 

 truncatus, and which he identifies, with some doubt, with Brogger's species. 



Horizon and Loral i firs. — Menevian : Solva; Dwrrhyd, near Solva. 



Section Longifrontes {addendum). 



Since the first part of this monograph was published I have found, amongst 

 the specimens collected by Mr. Rhodes for the Geological Survey, and at present 

 deposited in the Museum of Birmingham University, several heads and tails of 

 Brogger's A. incertus, of which accordingly I append a description. 



20. Agnostus incertus, Brogger. Plate III, figs. 1 — 3. 



1879. Agnostus incertus, Brogger, Nyt Mag. for Naturv., vol. xxiv, p. 70, pi. vi, fig. 4 a, b. 



1880. Agnostus incertus, Tullberg, Agnostus-arterna, p. 19, pi. i, fig. 6 a, b. 



Head rounded in front, with a narrow margin. Glabella narrow, bilobed ; 

 anterior lobe obtusely pointed in front ; posterior lobe long, nearly parallel-sided, 

 but with two scarcely visible lateral impressions on each side, elevated posteriorly. 

 Cheeks somewhat narrowed in front of the glabella, where they are separated by 

 a furrow. 



Tail sub-quadrate, with a narrow margin, which bears a minute spine on each 

 side. Axis broad and elevated in front, somewhat abruptly constricted behind, 

 and produced into a depressed and narrow point wedged between the lateral 

 lobes ; segmentation indicated only by two short grooves on each side of the 

 elevated anterior portion; in the space between these grooves is a prominent 



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