142 EIGHTH REPORT — 1838. 



angles, to the surface. In tlie longitudinal section of this tooth, the 

 twisted appearance, above described, of the peripheral calcigerous tubes, 

 was seen to be due to the number of side branches given off at an 

 acute angle to the main tube. At the apex the tubes radiate, and sud- 

 denly diverge to proceed transversely to the sides. In the body of the 

 tooth the main canals are surrounded by concentric lamellae, traversed 

 by radiating and anastomosing calcigerous tubes, which form a fine net- 

 work in the interspaces. 



Dictyodus, a sphyrenoid genus. — The body of the conical maxillary 

 teeth of this fossil species presents a beautiful assemblage of medullary 

 canals, having a general parallel course from the basis to the apex, divi- 

 ding and subdividing as they approach the latter, with interspaces gene- 

 rally equal to three or four of their own diameters, and anastomosing by 

 short branches crossing the interspaces, and thus intercepting quadran- 

 gular, sub-elliptical, pentagonal, or hexagonal spaces, elongated in the 

 axis of the tooth, but becoming shorter as they approach the apex, 

 which presents the appearance of a coarse irregular lace-work. The 

 interior of some of the larger canals is occupied with a granular matter. 



I have been able to detect the fine calcigerous tubes only at the cir- 

 cumference of the tooth radiating from the peripheral side of the su- 

 perficial canal into the clear enamel-like coating of the tooth. They 

 immediately begin to ramify at acute angles. 



The larger canals are continued directly from the coarse medullary 

 cells at the bony base of the tooth : the longitudinal ones are mostly 

 larger than the transverse or oblique short anastomosing canals. This 

 tooth resembles in general structure that of the Anarrhichas Lupus. 



The round pharyngeal teeth of the extinct genus Sphcerodus are 

 anchylosed to a bone of a cellular structure. The body of the tooth 

 consists of coarse tubes, which arise insensibly from the basis, where 

 they have a diameter of sjVo^^* <^f ^" inch, and proceed directly and 

 perpendicularly to the surface of the tooth. The characteristics of 

 these tubes are, first, that they are so closely arranged together, that 

 only one-fourth of their own diameter intervenes between them at their 

 origins. Secondly, they present the appearance of a closely-tvisted 

 bundle of smaller tubes, and begin immediately to give off short and 

 somewhat coarse branches at veiy acute angles ; these branches increase 

 in number, and the trunks proportionally diminish, until they have 

 traversed two-thirds of the vei'tical diameter of the tooth ; they resolve 

 themselves into fasciculi of extremely minute twigs, which interlace 

 together, and in many places dilate into, or communicate with, nu- 

 merous minute calcigerous ceils, and form so dense a layer as to inter- 

 cept the light, excepting towards the circumference of the tooth, and 

 consequently at the two extremities of the section, where only the 

 structure above described is visible. Several small twigs pass beyond 

 this plexus into the clear enamel-like outer layer of the tooth, in some 

 parts of which traces are perceptible of a plexus of still more minute 

 tubes, or stria, which gradually diminished until they escaped the 

 highest magnifying power employed in this examination. 



Lepidotus, — The pharyngeal teeth of some of the species of this 



