60 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



The hypostoma of Lichas is of a characteristic type, broad, with tlie incised poste- 

 rior margin and its two lappets reminding of Asaphus, with its large rounded, faintly 

 elevated median part near the anterior margin reminding of Illaenus, as also do the 

 lenticles of the maculae. The maculae are of a minimal size, scareely 1 millim. in dia- 

 meter and thence easily overlooked. Pl. IV figs. 46, 48. 



As the hypostomas in this genus generally are found detaehed it is often diffieult to 

 decide to which species they have belonged. So it is with that delineated in Pl. IV fig. 44 

 natural size. The two maculae are situated near the centre of the hypostoma, just below 

 the grooves which surround the large, globular disc. They are sunk in a little eavity, 

 rounded and entirelv covered by globular lenses (pl. IV fig. 46). 



In another detaehed hypostoma of an unknown species the macula is smaller and 

 the few leutieular globules larger than in the other, tig. 48. 



Lichas latifrons Angelin. 



PL IV tig. 49. 



The maculae, seen in a thin section from the inside, are reticulate, fig. 49, or of a 

 pattern exactly like that in the changed eyes or maculae of Illaenus and even Asaphus, 

 and thus indicating that it was composed of short prismatic lenses. The vertical section 

 of a macula of a specimen from another locality rather suggests the presence of large 

 sphaerical lenses than elongated prisms. The great thinness of the macula in contrast to 

 the exeessive thickness of the shell around the eye is seldom so evident as here. 



A specimen of an unknown species, found detaehed, has the maculae unlike the 

 other species, oblique with the inferior end directed inwards, placed on the top of a sort 

 of a little elevation, fig. 47. 



Megalaspides Brögger. 



This author has a figure of M. dalecarlieus in II pl. 1 fig. 1!) and also of an un- 

 na med species fig. 20, both showing globular maculae. In Holm's original description of 

 his Megalaspis dalecarlieus (Trilobiten des Phyllograptusschiefers Dalecarlieus figs. 8, !)) 

 the same hypostoma is also figured. This genus seems to have a closer affinity with 

 Ptychopyge than with Megalaspis. 



Megalaspis Ang. 



The following figures of hypostomas with maculae have been published. 



Megal. limbata by BröGGER I, tab. XII, fig. 10, and in II pl. 2. fig. 22, somewhat 

 differing from the first figure. 



Meg. planilimbata Brögger II tab. 2 tigs. 21, 21a. Both figures incomplete and 

 incorrect. 



