VERTEBEATES. 141 



GENUS DELTODUS, Agassiz. 



Deltodns, AGASSIZ, M8S., 1859. Pivcilidu-R aiibl.<rris; Messrs. J. MOERIS and G. E. UOBEUTS, 

 1802, Quarterly Jour. Geol. Soc. London, XVIII, p. !>4; Messrs. NEWBEKKY and WOBTHEN, 

 iscii, Illinois Geological Survey, II, p. %; Lord ENNISKILLEN, IHti'.i, Catalogue of Typo 

 Specimens, Geol. Ma^., London, VI, extract, p. 4; Dr. L. DE KONINCK, 1878, Pauvre du 

 Calcaire Carboniflre de la Belgiaue, II, p. 63; Dr. F. BIGSBY, 1878, Thesaurus Devonico- 

 Carboniferous, 1878. 



These forms have thus far been recognized as having probably 

 pertained to one and the same generic category, as follows : 



Mandibular posterior teeth sub-triangular or sub-spatulate in out- 

 line, arched and slightly spirally inrolled from within outward. An- 

 tero-lateral border with a slight concavity in its course from the inner 

 angle to the inrolled outer margin, basal portion channeled and ter- 

 minated below in a thin rim, bordered above by the inbeveled belt 

 which is enveloped in the coronal enamel, and partaking of its punc- 

 tate structure ; postero-lateral border obliquely converging from the 

 sub-acute posterior angle toward the outer margin, gently arched, 

 nearly vertical, basal portion channeled and denned above by a nar- 

 rower, more or less abruptly-rounded belt of coronal enamel; inner 

 margin sigmoidally curved, broadly arched from the obtuse anterior 

 angle round the base of the coronal prominence, with a concave 

 course thence to the sub-acute posterior angle. Coronal region pre- 

 senting a prominent anterior ridge culminating in a more or less 

 rounded crest near the median line, with gentle inclination to the 

 antero-lateral border, abruptly sloped into the posterior depression, 

 which is denned by the more or less upraised border forming the 

 posterior alate lobe. 



Mandibular median teeth cuneate or elongate sub-trapezoidal in 

 outline, relatively narrow, transverse diameter, longitudinally arched 

 and inrolled at the outer margin. Postero-lateral border with a 

 slight sigmoidal curvature and somewhat oblique outward and for- 

 ward course, basal portion channeled and terminating in a narrow 

 edge, coronal belt relatively wide, abruptly beveled to the narrow 

 ridge limiting the basal furrow, the articular border nicely co-adapted 

 to that of the conterminous posterior tooth ; antero-lateral border rel- 

 atively short, less oblique in its outward and forward course, basal por- 

 tion relatively narrow, channeled, limited above by the rounded, inbev- 

 eled enamel fold ; the inner margin is peculiar for the obliquity of the 

 anterior half and sudden deflection nearly at right-angles in round- 

 ing the base of the coronal ridge, with a slight concavity in its 

 course thence to the sub-acute posterior angle ; coronal region tra- 

 versed by a strong median ridge, culminating in a rounded crest, 



