COLOSTRA* ION. 31 



Colostracon, gen. nov. 

 (kqKos, decurtatui : oarpaKov, testa.) 



Testa inverso-conica aut decurtato-fusiformis, tenuis ; spira valde depressa, plana 

 aut immersa : anfraettis quaterni quini vel seni, confertim convohtli, a lateribus planati 

 et plus minusve insinuati, postice truncaii acute angulati et ad unguium aliquanto cari- 

 nati sutura perspicua divisi; ultimus nulla- si rwn in canalieulum at in rostrum tamen 

 porrectus : superficies lineis inerementi notata et per partes spiraliter striata aut leviter 

 sulcata: apertura rectiuscula, coartata, testes longttudini adcequans, ante medium 

 dilatata, antice angustata ; columella arcuata, edentula, probngata et paulum con- 

 torta, labio Iran! reflexo modice obtecta, peranguste rimulata; labrum simplex. 



Shell inversely conical or decurtate-fusiform, thin ; spire greatly de- 

 pressed, plane or immersed : whorls four to six, compactly convolute, later- 

 ally flattened and more or less insinuate, behind truncate, acutely angulate 

 and at the angle considerably keeled, separated by a distinct suture; the 

 body-whorl produced in front into what must lie styled at least a beak, if not 

 a proper canal: surface marked with lines of growth and on portions spirally 

 striated or lightly grooved: aperture rather straight, contracted, as long 

 as the entire shell, widened before the medial line, narrowed in front; 

 columella arcuate, without folds, prolonged and somewhat twisted, moder- 

 ately covered by the thin reflexed labium, very narrowly fissured ; labrum 

 simple. 



Absence of folds from the columella and the presence of transverse stri- 

 ations upon their exterior exclude these shells from Cylindrites, but suggest 

 their classification under Actceonina. To the latter genus they would cer- 

 tainly seem to belong, were it not that a short but distinct anterior beak 

 or canal is present in our typical species. And though from imperfection 

 of the specimens it cannot positively be asserted that a similar beak occurs 

 in the second species described by us, nor in that refigured from Fraas, 

 the combination in both of like peculiar and striking characters leads to 

 the inference from analogy that the beak can hardly be wanting in perfect 

 examples. 



Of the species of Actceonina known to us from actual specimens, not one 

 shows any indication of a beaked or channelled aperture. Loriol's figure of 

 his .1. Davidsoni, etage portlandien (Monogr. des Stages supeneurs de la 

 Formation Jurassique de Boulogne-sur-Mer. p. 4 1, PI. vi, figs. 8, 9, 1873), 



