Dr. Ivor Thomaa — Trilobife Fauna of Devon and Cornicail. 195 



The triangular cheek-areas rise to the level of the glabella, and slope 

 fairly steeply towards the latter, but more gently outwards to a narrow 

 groove bounding the marginal border of the head-shiehl. This border 

 is broad and is continued from the fairly angular genal extremities to 

 the glabella, where its outer margin appears to be continuous, as 

 viewed from the dorsal side, with the anterior limits of the frontal lobe. 



The eyes, of which the left one is preserved, are each situated upon 

 a broadly elliptical lobe at the anterior extremity of the cheek-area. 

 Eight circular facets are present, and are arranged in two sub-crescentic 

 rows concave to the antero-lateral margin of the head-shield ; there 

 are five facets in the outer and three in the inner row. 



Localiti/ and Horizon. — This form was found in the Upper Devonian 

 greenish-grey slates of AVhiteway Farm-yard nearldcford, Devonshire. 

 In association with it are found Bechenella tissheri, sp. nov., Phacops 

 [Trim.) tripartitus, sp. nov., Proetus spp., Posidojiomya ventisfa (Miinst.), 

 Piickiola sp., and Entomis serratostriata (G. «& F. Sandberger). These 

 beds were identified by Professor Ka5'ser some years ago as equivalent 

 to the Cypridinen Schiefer (9, p. 185 ; 20, p. 513). 



Remarks. — The same specimen is referred to in a previous paper 

 (16, p. 100) as being allied to Phacops {Trim.) cryptophthahnus, Emmr. 

 As cryptophthalmus has been variously applied by different authors 

 (17, p. 168), it is well to emphasize that its use in this case is in 

 accordance with Giimbel's interpretation (5, pi. a, figs. 7-9). Giimbel's 

 figure shows a marginal border which narrows more rapidly anteriorly 

 and a more angular antero-lateral margin of the glabella. Comparison 

 with a figure alone is unsatisfactory, since allowance has to be made 

 for imperfections in the drawing as well as for the different positions 

 in which the specimen may have been held. 



A resemblance also exists with Trimerocephalus cyclophthalmus, 

 K. Walther (21, p. 318, pi. xv, fig. 1), from the Styliolinen Schiefer 

 of Schaderthal. The latter has a more curved posterior margin of 

 the head-shield, more rounded genal extremities, apparently a less 

 elongated glabella, and more eye facets, which also differ in their 

 arrangement. 



The eye of this species is seen to consist of two sub-crescentic rows 

 concave to the antero-lateral margin of the head-shield, while that of 

 Ph. {Trim.) pentops, Ivor Thomas (16, p. 97, pi. iii, figs. 1-4), is made 

 up of one sub-crescentic row convex to the same margin. 



The question of the development of eyes in Phacops has been 

 discussed by Dr. J. M. Clarke in a valuable paper on the visual area 

 of Phacops rana, Green (2rt). Among other important conclusions he 

 finds that " a definite relationship exists between the number of lenses 

 of the eyes and the size (i.e. the age) of the animal" {2a, p. 257), 

 and that " the number of lenses increases from youth to maturity and 

 decreases from maturity to senility " {2a, p. 258). The deduction 

 that " the primary lenses probably appeared in a single or double row, 

 a visual line parallel to the margins of the orbital node " {2a, p. 261), 

 is of particular interest in the consideration of the above-mentioned 

 forms. Dr. Clarke, in a letter calling my attention to the above 

 paper, expresses the opinion that Trimerocephahis is particularly 

 interesting "in respect to its senile decline of visual surface". In 



