HARDWICKE'S SCIENCE-GOSSIP. 



109 



and P. mammillaris. The first has the central part 

 of the tooth not much raised, with the transverse 

 ridges all insensibly merging into the surrounding 

 granulated area ; in the second (fig. 82), the trans- 

 verse ridges and furrows are coarser and mostly bend 

 round on reaching the granulated area, producing 

 gyrations suggestive of the specific name ; in the 

 third (fig. Si), the central part of the tooth is raised 



British Museum. To whichever jaw this dental 

 armature belonged, its arrangement is obviously very 

 different from that of Cestracion, and if we were now 

 venturing upon innovations, instead of simply 

 recording the present state of this branch of Palaeonto- 

 logy, we should remove Ptychodus altogether from 

 the Cestraciont family, and endeavour to find a place 

 for it in proximity to some of the Rays. 



Fig. 82. — Tooth of Ptychodus J>olygyrus. 



Fig. S3. — Diagram illustrating arrangement of dentition of 

 Ptychodus. 



Fig. 84 —Jaw oi Acrodus Anningice. (After E. C. H. Day.) 



into a more or less prominent dome, and the 

 surrounding granulated area is characterised by the 

 delicacy of the markings, and the frequent presence 

 of radiating grooves. 



With regard to the disposition of the teeth in the 

 mouth, very little as yet has been ascertained, owing 

 to the fragmentary nature of most of the fossils ; so 

 much is known, however, as is represented in the 

 diagrammatic plan (fig. 83), which embodies the 

 information afforded by specimens exhibited in the 



One other extinct genus is usually referred to the 

 Cestraciontidae, — Plethodus from the English Chalk. 

 It is founded upon detached dental plates, which are 

 flat, and of somewhat irregular outline, with a 

 punctated grinding surface. 



( To be continued.) 



"Engineering" mentions weather vanes illu- 

 minated by electricity, and a trial suggested with 

 one twenty feet long, with a light at either end. 



