CROSSOPTERYGII. 71 



Sub-Order 1. Haplistia. 



The sub-orders of Crossopterygians are most conveniently 

 based on the condition of the median fins, and the first is as 

 yet only very doubtfully determined. It is defined as com- 

 prising those families in which the supports of the median fins 

 are in two simple regular series (i.e., baseosts above axonosts), 

 much fewer in number than the dermal rays. The only genus 

 which can be even provisionally placed here is the remarkable 

 Lower Carboniferous fish named 



Tarrasius. A small, elongated, laterally compressed fish, not known 

 to attain a greater length than 0'06 m. The membrane-bones of the head 

 and opercular fold are well developed, and their exposed faces coated with 

 ganoine. The notochord must have been persistent. The pectoral fins 

 are obtusely lobate, but the pelvic fins are unknown. There is one con- 

 tinuous median fin extending from the back to the anal region, supported 

 apparently by a double regular series of cartilages, more numerous than 

 the apposed arches of the axial skeleton. The abdominal region is naked, 

 while the caudal region is enveloped in very small, thick, quadrangular, 

 ganoid scales, which scarcely overlap but are closely arranged. The single 

 known species, T. problematicus, occurs in the Calciferous Sandstones 

 (Lower Carboniferous) of Glencartholm, Eskdale, Dumfriesshire. 



Sub-Order 2. Rhipidistia. 



In this sub-order the median fins are always more or less 

 subdivided by the process of concentration of supports, already 

 described in the anal fin of the Dipnoan genus Scaumenacia 

 (fig. 20, p. 22). In each of the dorsal and anal fins the proximal 

 supports (axonosts) are fused into a single piece, while the 

 development of the distal supports (baseosts) is variable, though 

 these are always much fewer than the comparatively delicate 

 dermal rays which overlap them (fig. 19, p. 22). The notochord 

 must have been more or less persistent, though some genera 

 exhibit completed ring- vertebrae. Each ramus of the mandible 

 comprises a series of stout, dentigerous elements between the 

 normal splenial and dentary ; and the dentary is also bordered 

 below by three or four distinct " infradentary " plates (fig. 56 A, 

 p. 73). 



