On the maculae of the hypostoma. 



We shall now tura our attention to the visual organs whieh Liljevall discovered 

 on the hypostoma of Brooteus. In doing this, we may bear in mind, tli.it the genera in 

 which we really have found lenses on the maculae are relatively few, hut that we shall 

 review the maculae in all genera, which we have heen able to examine, and try to show 

 that even most of these may, although in an inferior degree, have acted as visual 

 organs. At the same time some more details shall he given ahout the cephalic eyes for 

 comparing thein with the hypostomic ones. 



Acidaspis Murch. 



The hypostoma is of a peculiar type, deviating from that of the other genera, 

 squarish or rectangularly transverse, entirely without terrace lines and no maculae proper. 

 Barkande has given no less than nine different samples of these hypostomas. 



Acidaspis erenata Emmr. 



(Pl. I figs 1—6.) 



There are certainly no maculae of the same sort as in so many other trilohites and 

 the interiör side of the hypostoma does not bear the least traces of impressions whieh 

 might be taken as the reverse of the maculae and still more less as muscular impressions. 

 In fact, the total absence of such in Acidaspis militates against the interpretation of the 

 macula impressions as muscular scars. When the maculae fail, also the so called muscu- 

 lar impressions fail. Moreover we are able to see still clearer in this matter through 

 what we know about the structure and position of the extremely similar hypostoma of Apus, 

 of which we have given a description in the end of this memoir. This sceletal part of 

 Apus is without any connection with the surroundings, excepting at its anterior margin, and 

 is consequently movable in a direction outwards and upwards. Along that anterior mar- 

 gin it is fixed to the outside of the ventricle through three pair of muscles. three musc- 



