Frof. C. Lapu'orth — On Olenellus Callavei. 531 



between glabella and eye-lobe flat, and separated from the glabella 

 by an irregular undulating groove. Frontal limb narrow. Surface 

 of free cheek slightly convex, sloping inwards to the eye-lobe, and 

 outwards to the groove bounding the lateral margin. Thorax with 

 18 (?) segments. Axial lobe elevated; prominently rounded and 

 narrowing throughout. Each axial segment is well defined. It is 

 thickened posteriorly and laterally grooved ; and it bears centrally 

 a short and stout recurved claw-like spine, with a spreading base, 

 and a sharp ridge-like upper surface. The temninal segments bear 

 in addition two lateral rudimentary spurs, while the central spine is 

 much reduced in size. Pleural lobes, flattened for the first half to 

 two-thirds of their length ; and then gracf^fuUy curving and con- 

 tracting to the recurved falcate extremity. The proximal surface of 

 each is relieved by an oblique pleural groove. 



The associated pygidium is small, simple, almost semi-circular 

 in form ; with slightly converging lateral margins, straight and 

 shortened posterior edge, distinct central elevation, and anterior 

 groove. 



The test or body-covering of both head and thorax is marked 

 throughout by a raised fretwork of inosculating lines or ridges, 

 the pattern of which varies in different parts. 



Dimensions. — The larger fragments collected indicate a length of 

 about six inches and a breadth of about four inches. With the 

 exception of Olenellus (Holmia) Broggeri, Walcott, this form is the 

 largest species of the genus yet discovered. 



Comparisons. — Olenellus Callavei, as I have already more than 

 once pointed out,^ is most intimately allied to Olenelhis Kjerulfi, 

 Linnarsson, from the Lower Cambrian of Norway, and Olenellus 

 Brt^r^feii, Walcott, from the basal beds of the Cambrian of Newfound- 

 land. These three species appear to me to constitute a special sub- 

 generic (?) group, intermediate between Olenellus, Hall (including 

 Mesonacis, Walcott, and Olenoides, Meek). They all possess the 

 remarkable " interocular " processes and conspicuous dorsal spines 

 of the latter, but differ in the absence of the facial suture. They 

 agree with Olenellus (including Mesonacis) in the general form of 

 the head and of the glabella, and in the peculiar ornamentation of the 

 test; but differ in the absence of the great median or terminal spine. 

 From both groups they are strikingly distinguished by the great 

 development of the occipital process. In allusion to this common 

 characteristic feature, I suggested for the group the title of Cepliala- 

 canthus;^ but, as Mr. Walcott has pointed out, this term must give 

 way to Holmia, the subgeneric title published by Mr. Matthew ^ 

 in June, 1890, for Olenellus (Holmia) Kjerulfi, Linnrs., the first 

 discovered species of the group. 



Olenellus Callavei not only agrees with 01. Kjerulfi in its general 

 characters, but in the conspicuous development of the interocular 

 spines, in the existence of a faint ridge sweeping back from the 



' Geol. Mag. etc, loc cit. supra. Fauna OhneUiis Zone, pp. 640-641. 



2 Geol. Mag. 1888. p. 641. 



3 Matthew, Trans. Roy. Soc. Canada, 1890, p. 160. 



