VERTEBKATES. 219 



tooth is laterally expanded, dividing the articular border by a more 

 or less conspicuous angulation into two distinct areas somewhat 

 variable in extent. The basal portion of the tooth is very deep, the 

 outer border considerably expanded beyond the coronal fold, more 

 or less produced at the posterior angle, and inbeveled below to the 

 relatively reduced inferior surface ; inner face broadly channeled and 

 interrupted by the before-mentioned angulation; anterior face also 

 excavated vertically and somewhat inbeveled from the plane of the 

 coronal surface ; posterior margin presenting a shallow vertical con- 

 cavity, the edges all round making an angle, except along the outer 

 side, with the inferior surface, which latter shows a faint median 

 ridge. The coronal surface presents a minute punctate structure, 

 the pores somewhat widely spaced, sometimes confluent, and enclosed 

 in the usual vertical prisms. Toward the posterior margin there 

 appear indications of transverse rugse. The basal walls are verti- 

 cally and irregularly plicated, and perforated by the irregular pores 

 and occasional alviolar cavities; the abraded inferior surface shows 

 delicate vermiculose markings, but in the perfect state it presents 

 the usual dense, smooth layer, which also envelopes the walls of 

 the base. A large-size perfect tooth measures across the posterior 

 margin 32 mm., and about 25 mm. across the anterior extremity ; 

 length along inner border 45 mm. ; depth of tooth at the posterior 

 margin 21 mm., and 20 mm. at the opposite extremity. 



The collections afford four or five examples of a form of teeth, 

 which, in their coronal contour and general conformation, also in 

 the details of their superficial structure, show unmistakable relation- 

 ship with Psammodus, although they present a type differing in cer- 

 tain well-defined characteristics from the previously herein described 

 species of the genus. The first authentic example of the form here 

 alluded to is that described by Professor McCoy 1 under the name P. 

 canalicvlatus, from the mountain limestone of Ireland. The examples 

 before us all belong to the same side, while that figured by Professor 

 McCoy represents a tooth from the opposite side. As in the pre- 

 viously described forms, the character of the articular walls clearly 

 indicate the paired arrangement of the teeth, indeed in the instance 

 of the Upper Burlington species, P. Spring eri, the examples from 

 opposite rows are about equally represented in the collections, and 

 the same may be said of the European species, P. porosus and P. 

 rugosus (sp. ?) of Agassiz. While this statement is true so far as it 

 goes, and the form in question doubtless occupied a lateral position 



1 British Palaeozoic Fossils, 1855, p. 643, PL 3 G, f. 12. 



