64 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITEN. 



niaculu', near the exteriör angles of the horder groove are pear- or spoon-shaped with the 

 apex direeted downwards and outwards, and in consequence the longitudinal axis quite 

 opposite to that of the usual direction. The surface is sniooth, a little concave. No 

 structure has heen discovered. From the pointed apex a slightlv elevated ridge rims 

 along the posterior horder of the hypostoma and joins the apex of the other inacula. 



Paradoxides Brongn. 



The oldest record of any hypostoma at all amongst the trilol)ites is that given hy 

 Wahlenberg in his Petref. Suecana (1818) p. 37 Tah. 1, fig. 6, which he, however, con- 

 sidered as the head of the trilobite which he named Entomostracites bucephalus. Figures 

 with maculae have been given of 



Par. Davidis Salter Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine figure of a 

 hypostoma with two large oblique maculae. LiNNABSSON in »de undre Paradoxideslagren 

 ^ id Andrarum» pl. II fig. 2 delineates a gigantic hypostoma with two crescentic tubercular 

 maculae, having 10 mms in length. 



Par. Forchhammeri Angelin Pl. II fig. 3. Brögger, Paradoxidesskiffr. vid Kraekling 

 tah. II fi<>\ 10 two marks somewhat different from Angelin's figure. Linnarsson 1. c. 

 pl. I figs. 9, 10 two hypostomas with maculae. 



Par. Tessini, var. Wahlenbergi Ängel. Pl. la, fig. 1 b. 



Par. Tessini, var. Oelandicus Ang. Tah. I a, fig. 2 b. Angelin's figure of this 

 hypostoma is incoinplete in so far that he had not found the peculiar falciform horns 

 on both sides of the posterior horder. Pl. V, fig. 33. 



Par. ruguloaus Corda has, according to Brögger, Kraekling, pl. II fig. 2 two long, 

 straight callosities, probably corresponding to the maculae in other Paradoxida'. 



Ciir. Boeck gives in Mag. for Naturvidenskaberne Bd 8, 1828, in his paper on the 

 Trilobites on the plate fig. 16 the hypostoma of a Bohemian species with well developed 

 concave maculae, probably a cast. He compares it with the Entom. bucephalus of Wahlen- 

 berg, hut says he cannot explain its natur*'. 



The maculae of P. oelandicus, pl. V, fig. 34, are tubercles, oblong, sniooth, and 

 have invariably an oblong scar along the centre, where probably once a thinner membrnne, 

 as in the cephalic eyes of Calynimene, contained the lenses. 



Peltura M. Edw. 

 Peltura scaraba^oides Waulenb. 



Pl. Ill tift'. 4-2. 



Wc have given a figure of the hypostoma, showing two large, elongated tubercular 

 maculae or rather resemblances of such, ;is they are cové!red with tcrrace lines ;is well as 

 i he rest of the surface. They have, however, the same position as rea] maculae might 

 have, hut are direeted in ;i line parallel with the longitudinal axis of the hypostoma. 

 The cephalic eyes have been described above at page 29. 



