Manchester Memoirs, Vol. Ix. (1916), No. 2. 19 



Trisiichopteriis alatus, Eg. 



The fine series of Tristichoptem's alatus collected by 

 one of the authors at the Deerness Quarry, on the main- 

 land of Orkney, shows some features hot displayed in 

 those which Egerton and Traquair figured. One indi- 

 vidual, No. L. 8429, a very large fish dorso-ventrally 

 crushed and viewed from the ventral surface, has had the 

 cheek plates of the left side freed from the top of the 

 skull, and turned outwards on the maxilla as an axis. 

 They are now represented by the beautifully sharp im- 

 pressions of their visceral surface. This shows quite 

 clearly that, in addition to the large cheek plate described 

 by Traquair as a pre-operculum, there are present a real 

 pre-operculum and a small quadrato-jugal. The pre- 

 operculum is of interest because it shows a ridge on the 

 inner surface, which no doubt gave support to the 

 hyomandibular. Except for the much larger size of the 

 squamosal, the cheek region much resembles that of 

 Rhizodopsis. Another specimen shows the endoskeletal 

 supports of all the fins except the first dorsal. These 

 agree very closely with those already described by Eger- 

 ton and Traquair and very strikingly with those which 

 are well known in EustJienopteron foordi. A third speci- 

 men is remarkable for the beauty of preservation of the 

 fins of the posterior half of the body. It shows that all 

 the median fins and the pelvics are far larger than they 

 are represented in Traquair's familiar restoration, the 

 second dorsal and anal reaching to a point half-way 

 between the origin of the caudal and its termination. 

 This specimen also shows that the dorsal and ventral 

 margins are produced in a way quite similar to that which 

 characterises Eusthenopteron. 



