42 LUNDSTRÖM, VISUAL OEGANS OF THE THILOBITES. 



Asaphus sp. 



Pl. I Hg. 27—30. 



from the islet of Sandö, north of Gotland. Although \vo cannot give an v account of ita 

 hypostoma, not having had sufficient material, the structure of the eyes is so peculiar 

 that it seems worthy of being recorded. The integument is extremeh" thin and trans- 

 parent and the subjacent lenses are clearly seen, and throngh their impact on the thin 

 integument they make this to stånd out in a verv low relief above them (fig. 27). These 

 lcnscs are uncommonly short, forming at the surface rather oblong, slightlv hexaedral 

 prisms with a narrow interspace between them. In a section lower down they have the 

 shape of hollow, white rings filled with black mud and in a longitudinal section the white 

 walls of the lcnscs look like short pointed spikes and interiorlv they are completelv empty. 

 \\ c here tind also the same gradual change from regular cones to the spongious börder 

 zone as in the above raentioned species f. 29. The börder zone is finely reticulated. 





Asaphus (Isotelus) gigas J. Mali.. 



Pl. II tigs. 1—3. 



The enormous hypostoma of this giant resembles in a high degree that of the Asa- 

 j)hi, hut is at the same ti me the most evident verification of the exj)erience that the 

 hypostoma per se cannot be regarded as the sole criterion for determinating the generic 

 affinities of different species. The whole structure of this trilobite in other respects gives 

 it a quite independent position, distinet from Asaphus. 



We have had at our disposal several specimens of the large hypostoma, the dimen- 

 sions of the largest, fragmentary in its anterior margin, being as follows: breadth 48 mm. 

 Icngth 41 mm., probably 47 mm. when entire, breadth of each of the posterior lobes at 

 their bases 19 mm., length of the same 23 mm. The macuUe which are placed on the 

 tlat surface of the hypostoma without being at all elevated, are prominent through their 

 great size and their påle, whitish colour. They are somewhat oblongue having their 

 longest axis directed inwards and downwards. They attain a diameter of four millimeters. 

 Horizontally sectioned, f. 2, they show the spongious texture and vertically the quasi pris- 

 matic reticulate structure richly developed, f. 3. The pilla rs are very distinet and con- 

 tinue without interruption all through the macula and in the interstices there are traverses 

 joining between two pillars or partially rilling the darkish interspaces, tlius giving the 

 whole the aspect of some tabulate» coral sectioned. By the inspection of the horizontal 

 section alone, presenting the irregular spongy texture it would have been impossible to 

 imagine the ordinated arrangeraent which the vertical section reveals to us. The eyes 

 consisl of regular hexaedral prisms, and there is a very sharp boundary line between 

 iliein and the spongious /.ene, which is very narrow and without distinet separating lines 

 joins with the cephalic test. For the rest there are in all probability at least two different 

 species scnl from America under the nainc of Isotelus gigas, of which only the largest, 



