4tf LINDSTRÖM, VISUAL ORGANS OF THE TEILOBITES. 



B. sulcatus Ldm. 



Pl. II figs. 33—40. 



The hypostoma is broadly shieldformed and the anterior margin is most charaeteristic 

 being in its central part elongated in a short evenly rounded projection. The side wings 

 are triangulär acuminate, bent n little backwards and towards the interiör surface. There 

 is only :i single rounded field passing into an evenly bent börder, in front of which the 

 projecting, elliptic macula> lie. A second pair of wings stretches backwards near the 

 posterior margin. We have not obtained any sections of the maculse. 



The cephalic eyes figs. 36 — 38 are covered with an uncommonly thick stråt um of 

 homogenous shell, one and a half times as thick as the subjacent lenses. These are com- 

 paratively broad, above passing indistinctly in the covering stratum, below, at their basis, 

 convex. In a horizontal section figs. 33, 34 they exhibit the common hexaedral or penta- 

 edric aspect, their exteriör sides being white and confluent without any separating lines. 

 Their interiör is dark and from the polygonal sides white lines go in giving the whole 

 the appearance as of a starry, composite eoral with its septa. The correspondance in 

 structure with Bronteus laticauda pl. II fig. 7 is striking. A narrow space separates the 

 surface of the eye from a nearly similar patch below the eye on the cheek 35 b, 38 l>. 

 But there the position of the composing strata is singularly reversed and the spongious 

 or pseudo-prismatic stratum lies elose to the outside and the homogenous stratum on the 

 inside. The white longitudinal lines are well marked out and regular in the vertieal 

 section, fig. 38 b, indicating the prisinatic structure most clearly. 



B. barriensis Mdrch. 



Pl. II figs. 31, 32. 



The hypostoma fig. 31 is of so transverse a shape that the breadth much sur- 

 passes the height, the former being 6 millim., while the latter attains only 3,r. millim. 

 The anterior margin is so much arcuated and sloping backwards that it forms an obtuse 

 angle. The side wings are triangulär and acuminated and from them a thick pad rims 

 backwards and forms the posterior margin. Below the largest field of the exteriör sur- 

 face the maculre rest on a ridge having a much elongated, vermiform shape and sunk in 

 the surface (fig. 32) a little more than three times longer than broad. In its shape it ap- 

 proaches much to the hypostoma of Bum. insignis Hall as delineated by Salter pl. 27 fig. 7. 



In another, undetermined Bumastus (pl. II figs. 47, 48) ' the hypostoma (from 

 Klints, Othem, isle of Gotland) is of a more elongate obovate form, the anterior margin 

 elevated, rounded, the terrace lines faint, the tubercles are larger than in the former 

 Bumasti, oblong, curved, beanshaped, evenly rounded and smooth. 



1 Probably tlio same as »Bum. sulcatus» pl. XII fig. 12 in my paper on ilio Trilobites of Gotiaud 1885. 



