PRESIDENT'S ADDRESS. lix, 



those on the hinder part of the jaws are molar-like. Sir Philip 

 Egerton describes the scales as being regular, lozenge-shaped, 

 each bearing on its inner posterior margin a thick, solid bony- 

 rib, sliced off obliquely above and below on the opposite sides, 

 forming splices with the corresponding processes of the adjoining 

 scales. 



Mesodon, Wagner. The head and opercular-bones externally 

 ornamented with granulations ; vomerine teeth arranged in five 

 longitudinal series, while the splenial are usually arranged 

 irregularly. M. damoni is a large species, known by the dentition 

 of the lower jaw ; the principal series on the splenial bone 

 consist of teeth twice as broad as long. It has been found in 

 the Portland Limestone, Upwey. 



Microdon. The body of this fish is almost discoidal, with a 

 small abbreviated caudal pedicel. The scales of the anterior 

 part of the body are ornamented with rugae and pittings. 

 M. radiatus, Agassiz, is frequently found in the Purbeck Beds of 

 Langton, near Swanage. M. pagoda, Blake, in the Portland 

 Stone (flint-bed), Ridgeway, near Weymouth. 



Gjrodus, Agassiz. The body of this fish is similar in shape to 

 the last ; the teeth are rugose and mammillated. Those of the 

 vomer are arranged in five, and the splenial in four series. 



G. cuvieri, Agassiz. This is a large species, known only by its 

 dentition. It has been found in the Kimmeridge Clay at 

 Weymouth. 



G. coccoderma, Egerton. A species only known by the denti- 

 tion. The vomer is furnished with a prominent median series of 

 teeth ; the outer row of the series are obtusely conical. There 

 are two fine vomers of this species from the Kimmeridge Clay of 

 Kimmeridge Bay in the Geological Department of the British 

 Museum which I presented to the Trustees. They were 

 described and figured by the late Sir Philip Egerton in the 

 Quarterly Journal of the Geological Society, 1869. One is 

 erroneously named Sphcerodus gigas. 



Aspidorhynchidae, Aspidorhynchus, Head and trunk much 

 elongated, the snout projecting in advance of the lower jaw. 



