of Edinburgh , Session 1874-75. 
513 
The following Communications were read : — 
1. On the Physiological Action of Light. Part II. By 
James Dew r ar, Esq., and Dr John G. M'Kendrick. 
2. On the Structure and Systematic Position of Tristi- 
chopterus alatus, Egerton. By K. H. Traquair, M.D., 
F.G.S. 
The cranial osteology and the dentition of Tristichopterus have 
been hitherto entirely unknown, and we were but imperfectly 
acquainted with the structure of the pectoral fins. Consequently 
great doubts have prevailed with regard to its affinities, though it 
was supposed to be allied in many respects to Dipterus. A suite 
of specimens from John O’Gfroat’s, in the Edinburgh Museum of 
Science, collected by Mr Peach, the original discoverer of the fish, 
subsequent to the publication of Sir Philip Egerton’s description, 
throws great light on the previously unknown points of its struc- 
ture, as well as on its affinities. In the osteology of the head it 
presents a striking resemblance to the Saurodipterini , and to the 
genus Gyropty chius, as described by Pander. The teeth are acutely 
conical, and of two sizes, large and small; the larger teeth have 
their bases fluted externally, and internally the dentine is seen 
to be thrown into a series of simple folds, the pulp cavity becoming 
simple towards the apex of the tooth. The shoulder girdle is 
provided with interclaviculars; the pectoral fin is subacutely lobate. 
The structure of the head, the dentition, and the form of the 
paired fins, show that Tristichopterus has nothing whatever to do 
with Dipterus. It seems to be more closely allied to Gyroptychius 
than to any other known genus. 
Tho following Gentleman was duly elected a Fellow of 
the Society: — 
John Aitken, Esq., Darroch, Falkirk. 
