STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 379 
joins it in front, and is then placed along the upper sloping margin of the 
maxilla, which it overlaps, while internally it is in contact with the outer 
margin of the anterior frontal in front of the orbit. 
So far as they can be deciphered, the heads of the other species of the genus 
seem to agree in their osteology in all essential points with that of P. parvulus, 
as described above. 
The dentition of the true Platysomi is quite unlike that of Hurysomus, Young, 
or of Globulodus, Miinster. We have already seen that Acassiz states that the 
jaws of Platysomus were armed with “petites dents en brosse trés-pointues,” 
and that in P. gibbosus “ ou apercoit quelques petites dents au bord du 
maxillaire,” although he afterwards suspected the identity of Globulodus with 
the present genus. . Professor Youne states that the jaws of P. parvulus are 
“armed with slender conical teeth, those in the lower slightly larger and more 
distant than those in the upper jaw.” Messrs Hancock and Arruey describe 
the dentition of P. rotwndus in the following terms :—“ The mandibular teeth 
are minute, conical, and pointed ; those of the maxillaries are of the same 
character, but more minute ; on the premaxillaries they seem a little larger.” 
And regarding P. Forsteri the same authors proceed to state—‘‘ The mandibular 
teeth are large, conical, stout, and obtusely pointed ; those of the maxille are 
small, conical, and tubercle-like, with wide bases and recurved apices, and are 
disposed without order along the alveolar border.” 
So far as my own observations go, I have only seen the teeth clearly as 
they exist in the maxillary bones of P. Forstert and P. parvulus. As regards 
the former species, I can certainly corroborate the description given by Messrs 
Hancock and Atruey. There the maxillary teeth resemble small tubercles, 
and are irregularly arranged in a narrow band, which passes along the inner 
aspect of the bone just above the lower margin. In Pl. parvulus (Pl. VI. fig. 11) 
the appearances are somewhat different; the teeth being arranged in one row 
on the lower margin of the maxilla. They are nearly equal in size and exces- 
sively minute, requiring a strong lens for their examination; in shape they are 
cylindrical, becoming slightly enlarged towards the apex, when they become 
suddenly and rather obtusely pointed. The expression “slender conical” used 
by Professor Youne in reference to the teeth of this species, would I think tend 
_ to recall the form of tooth prevalent in the Paleeoniscidze, whereas in reality they 
more resemble those of Achmodus ; there is also no very material difference 
between these maxillary teeth of P. parvulus and the mandibular ones of 
Mesolepis. 
Very little has been said as to the endoskeleton of the trunk in the genera 
of Platysomide already examined, the internal bones being always more or less 
obscured and hidden from view by the thick outer covering of scales. Such 
strong glimpses, however, as we do occasionally obtain lead us to suppose that 
VOL. XXIX. PART I. 5 D 
