ANGELIN Palæontologia Scandinavica = ANG. BARRANDE Système silurien de Bohéme = BARR. BRÖGGER Die silurischen Etagen 2 & 3 im Christianiagebiet = BR. 1. ---- Die Ausbildung des Hypostoms bei einigen Asaphiden BR. II. HALL, JAMES, Palæontology of N. York, vol. I, II, VII = H. HOLM Trilobiter ur Dalarnes graptolitskiffer = HM I. ---- Ostbaltische Trilobiten, Illæniden, = HM II. NOVÁK, Studien an Hypostomen Böhmischer Trilobiten. Three different papers between 1884-1886. = N. II, N. III, N. IV. SALTER Monograph British Trilobites. SCHMIDT Ostbaltische Trilobiten I, II, IV. = SCHM. WOODWARD, HENRY, Carboniferous Trilobites. Pal. Soc. 1883-84. * * * * * [Illustration: _Hypostoma of Bronteus, front and side views._ _a._ anterior margin. _b._ posterior margin. _c._ anterior pair of wings. _d._ posterior pair of wings. _e._ anterior groove. _f._ posterior groove. _g._ lateral grooves, _h._ maculæ.] Before proceeding further I shall give a general representation of the shape of the hypostoma. This enigmatic part of the trilobite skeleton resembles as to its outline a heraldic shield being broad at its anterior margin, gradually tapering towards the posterior extremity, which generally is acuminated or, as in the Asaphidæ, bifid. In a few, as Deiphon, Remopleurides, it is nearly square. Its exterior surface may consist of a single field or he divided transversally into two or three fields, according to the presence of one or two shallow grooves. These are transverse, curved forwards near the lateral margins and may be designated as the anterior groove and the posterior groove. Beside these grooves there are in some instances others, one on each of the lateral margins, parallel with these. On both sides of the anterior margin a protuberance, sometimes of considerable length, stands out. They are bent backwards towards the interior side of the hypostoma and in some genera, as Dysplanus, assume the shape of long, acuminated, halfways hollow horns. These protuberances were called »_ailes_» by BARRANDE and it is best to retain this term, in english »wings», in spite of its being not quite expressive. A smaller posterior pair lies nearer to the posterior margin and these wings are so much bent backwards, that they are seldom visible from the outside. They are prolongations from the »duplicature» or the narrow fold of the posterior margins round the interior side of the hypostoma. For the rest the whole exterior surface is covered with terrace lines running concentrically and varying according to the different genera and even species. Many other genera, again, as Acidaspis, Calymmene, Homalonotus, Dalmanites etc., are devoid of them and instead granulated or smooth.