886 DR RAMSAY H. TRAQUAIR ON FOSSIL FISHES COLLECTED BY THE 



the nature of opercula. That they are homologous with the pectoral flaps of TJielodus 

 and Lanarhia I still believe, but that they are of the nature of pectoral fins is another 

 question, the consideration of which must be left for the present. 



Finally, as the general resemblance between Ateleaspis and Cephalaspis is now 

 seen to be so close, I no longer see the necessity of allotting a distinct family to the 

 former genus, and propose therefore to classify it henceforth as a member of the 

 Cephalaspidae. 



Order ANASPIDA. 

 Family Birkeniid^e. 



Lasanius problematicus, Traquair. 

 PI. II. figs. 4-8. 



A considerable amount of fresh and interesting information is available, and 

 accordingly more than one-half of Plate II. is occupied with figures illustrating new 

 or little-known details of configuration and structure of this still problematic little fish. 



In my previous memoir I showed how that in many specimens of Lasanius the 

 general form was indicated in parts by a carbonaceous film, and how that in one 



Fig. 4. — Restored outline of Lasanius problematicus. 



example (L. armatus, PI. V. fig. 12) the rays of the caudal fin were preserved, proving 

 that that fin was heterocercal in structure, and thereby giving a clue to the position of 

 the median row of scales, which accordingly turned out to be ventral, and not dorsal as 

 I had originally supposed. 



Since then a number of fresh specimens have been collected at Seggholm by Mr 

 Tait, in which the general outline is still better displayed, as will be seen on inspecting 

 Plate II. figs. 4-7 of the present paper. The constancy in the examples there figured 

 of a feature, which I looked upon as merely an accidental one in the two specimens 

 figured in my previous memoir (PI. V. figs. 6 and 12), induces me to produce a new 

 restored outline of Lasanius (text figure), embodying a modification of the contour of the 

 anterior part of the creature. It will be seen that in all these specimens the greatest 

 depth is at the region of the post-cephalic rods, and that in front of these the dorsal 

 outline suddenly falls or slopes away down to a distance of about half the depth of the 

 body, and then passes forwards again nearly horizontally into the outline of the bluntly 

 pointed head, which is thus only one-half the depth of the body a little way back. 



