

54 



LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



around the lenses and the membranes of two contiguous lenses sometimes lie so close, 

 that they entirely till up the interspace between the lenses, leaving only a narrow slit 

 between them raarking their boundaries. The space between the lenses, tig. 46, is porous, 

 being perforated by tubes, vvhich continue vertically down through the shell of the check. 



Diihn. sclerops Dalman. 



Pl. IV fig. 3. 



Commonly this species has been ranged with Phacops, but it is evident that it as 

 to the conformation of ita glabella and the head in general is highly discrepant, and in 

 these respects is more concordant with Dalmanites, though still deviating as for instanee 

 in the pygidium and some details of the head. Therefore it may be jostified with Friedr. 

 Schmidt l to consider it as a generic division, though not, as he has it, as an independant 

 genus, but as a subgenus of Dalmanites. 



The granulated hypostoma differs from that of Dalmanites proper in liaving the an- 

 terior margin strongly arched, the pointed wings almost on the median line of the hypo- 

 stoma. There is one groove near the posterior börder and the maculae lie as two, narrow 

 long, crescentic grooves close below the wings. The few sections give no clear idea of 

 any structure. 



The cephalic eyes are constructed on the same plan as in the previous species, the 

 lenses being surrounded by a frame. 



Dalm. vulgaris Salter. 



Pl. III figs. 47—52. 



The hypostoma has the common shield-like shape, fig. 51, anteriorly slightly areu- 

 ated, the short obtuse wings curvated towards the interiör surface. It is finely granulated. 

 The groove does not go so near the cuspidate posterior margin as in other species. The 

 two niacula?, tig. 52, are narrow and elliptic, somewhat convex and lie in a little con- 

 cavity, a narrow sniooth space around is devoid of the granules, which cover the surface 

 of the rest. They show absolutely no structure, excepting a few indistinct blackish spöts. 



The cephalic eyes have been of ten described and delineated. Most detailed are the 

 descriptions and figures given by Salter in the Menioirs of the Geological Survey Dec. II. 

 pl. 1. In the figure 4 a part of the surface is shown in well preserved state. He says 

 that the cornea is . . . present and distinctly convex över each lens, the intermediate 

 portions being ornamented with tubercles and granules». He thinks also that there are 

 undeveloped or small lenses between the larger. His fig. 5 represents the frames around 

 the lenses, these being probably löst. His explanation of tig. 6 I cannot understand. 

 There are weathered or spoilt lenses figured. 



There is no doubt that the granulated surface of the head continues between the 

 more or less free lying lenses as shown here in a new figure taken from a specimen 



1 Revision, I, p. 76. 



