54 Sir Philip Grey-Egerton — On some New Pycnodonts. 
distinct from any otlier of the genus. I have named it after its 
discoverer and forrner possessor, Dr. Bowerbank, who I am liappy to 
know is still able to take bis share of scientific work in those brancbes 
of research in wkich he has laboured so kard and done so much. 
4. Ox A VoMEKINE PLATE OF A PyCNODCS FKOM THE LONDON CLAY. 
Sheppey. PI. IY. 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 maxillary apparatus in this family. 1 
Häring 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 
fixe 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 (PI. IY. 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 IY. Fig. 1 n) of the nasal (Etkmoid) 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 
wide, to which the premaxillary bones were attached. This structure 
has been well described by Thiolliere, who remarks, “L’ethmoide est 
remarquable par l’etendue de la cloison verticale qu’il presente au- 
dessous du front, et au-dessus du sphenoi'de 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 
forrned by the frontal line and the palate is sixty-seven degrees. 
Althougk I have stated that the nasal bone is united by suture to 
the vomer, this mode of attackment 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 skallow 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 incbes 
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 affords 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 b) 
has four teeth in order, the two next wanting, and the front one in 
situ. On the left side (PL IV. Fig. 2 c) tkere are six teeth, all 
regulär. All the teeth in these rows are oval and arranged obliquely 
1 Thiolliere is inclined to think that the so-called Yomer may he compounded of 
three bones, the Yomer proper carrying the three median rows of teeth, and the 
Palatine or Maxillary bones the outer or marginal rows . — Poissons Fossiles du Buyey, 
p. 18. 
