V. Notice of the Fossil Fishes found in the Old Red-Sandstone formation of 

 Orkney, particularly of an undescribed species, Diplopterus Agassis. By 

 Dr T. S. TRAILL, F. R. S.E. 



(Read 21st December 1840.) 



It is well known to those who have paid attention to the progress of Fossil 

 Ichthyology that, until the publication of M. AGASSIZ, the distinctive characters 

 of the orders, genera, and species of Fossil Fishes were but imperfectly under- 

 stood. Vague analogies were relied on to connect them with the types of living 

 genera, and the looseness of the received specific characters rendered it difficult 

 for the geologist to determine whether the specimens he collected were previously 

 recognised, or still nondescript. It is obvious that useful characters of fossil species 

 are chiefly to be obtained from those portions of their structure least subject to al- 

 teration from decay ; and as the exterior scaly envelopes of the primeval fishes are 

 usually the portions best preserved and most easily recognised in their rocky se- 

 pulchres, M. AGASSIZ was naturally led to study these with minute attention. This 

 acute observer speedily discovered that, in the form and connections of the scales, 

 he had a general character which would enable him to connect into very natural 

 groups, species differing from each other in size and form. On this basis he has 

 established his four Orders of Fossil Fishes the Ganoidei, the Placoidei, the 

 Ctenoidei, and the Cyclodei divisions named from the appearance of the scales. 



The bones of the body, especially of the head and the teeth, are often found 

 in a state of high preservation, especially in our schistose rocks ; in the layers 

 of which the general form of the specimen is easily recognisable. M. AGASSIZ has 

 subdivided his Orders into several Families, also natural groups, founded chiefly 

 on the form and position of the teeth, the disposition of the scales around the 

 body, the osseous or fibrous structure of the skeleton, and the general form of the 

 body of the fish. 



These Families, judging by then* living analogies, present other natural 

 groups, which he has considered as genera ; the principal characters of which 

 are drawn, as in existing fishes, from the number, form, and position of their fins, 

 which are often preserved, even in the most delicate articulations of their rays, 

 with wonderful precision, from the structure of the tail, the principal organ of 

 progression, from the arrangement of the teeth, the form of the bones of the 

 head, and the manner in which the vertebral column is terminated. 



VOL. XV. PART. I. B b 



