﻿54 Sir P/nlijJ Gi'oy-Egerton — On some New Pycnodonts. 



distinct from any other of tlie genus. I have named it after its 

 discoverer and former possessor, Dr. Bowerbank, who I am happy to 

 know is still able to take his share of scientific work in those branches 

 of research in which he has laboured so hard and done so much. 



4. On a Vomerine Plate of a Pycnodus from the London Clay, 

 Sheppey. PL IV. Figs. 1, 2. 



I have long had in my possession a Pycnodont vomer, found in 

 the Eocene Clay of the Isle of Sheppey, which first gave me a clue 

 to the anatomical details of the maxillai'y apparatus in this family.' 

 Having been found in clay, the matrix was easily cleared away so 

 as to display the entire bone, in so far as it was preserved. The 

 five rows of teeth, forming the roof of the mouth in all the species 

 of Pycnodus, are implanted on the under surface of the vomerine 

 plate, which is fully one inch in width. A solid vertical plate of 

 bone (PL IV. Fig. 1 v) ascends from the upper surface of the vomer 

 to the height of one inch, where it meets a corresponding descending 

 plate (PL IV. Fig. 1 n) of the nasal (Ethmoid) bone, to which it is 

 united by suture. The anterior margins of both bones expand 

 transversely so as to form a frontal shield seven-tenths of an inch 

 wdde, to which the premaxillary bones were attached. This structure 

 has been well described by Thiolliere, who remarks, "L'ethmoidc est 

 remarquable par I'etendue de la cloison verticale qu'il presente au- 

 dessous du front, et au-dessus du sphenoide anterieure et du vomer." 

 The total height of the frontal profile is two inches and a half, but 

 the upper portion of the nasal bone is broken away. The angle 

 formed by the frontal line and the palate is sixty-seven degrees. 

 Although I have stated that the nasal bone is united by suture to 

 the vomer, this mode of attachment is confined to the frontal union. 

 Two thin lateral plates of bone expand downwards from the sides 

 of the nasal bone and overlap the vomerine walls to within half an 

 inch of the dentary platform. The space thus left of the vomer has 

 the appearance of a shallow groove, and this groove (PL IV. Fig. 1 o) 

 at its distal extremity is arched over by a fragment of bone (PL IV. 

 Fig. 1 s), probably an anterior process of the presphenoid, forming 

 a large foramen, which in all probability constitutes the olfactory 

 duct. The dentigerous platform of the vomer measures two inches 

 and a half long by one inch and four-tenths wide. The median 

 row (PL IV. Fig. 2 a) counting from behind, has four large oval 

 teeth in succession, then comes a smaller circular tooth, succeeded 

 by a still smaller one, and in front a tooth of the normal form and 

 size. This afi'ords another and striking instance of the irregularity 

 in the tooth development in the Pycnodont family to which I have 

 before referred. The second row on the right side (PL IV. Fig. 2 h) 

 has four teeth in order, the two next wanting, and the front one in 

 situ. On the left side (PL IV. Fig. 2 c) there are six teeth, all 

 regular. All the teeth in these rows are oval and arranged obliquely 



1 Thiolliere is inclined to thini that the so-called Vomer may be compounded of 

 three bones, the Vomer proper carrying the three median rows of teeth, and the 

 Palatine or Maxillary bones the outer or marginal rows. — Foissons Fossiles du Bugey, 

 p. 18. 



