GREGORY: FISH SKULLS 201 



to Boulenger (1910, p. 623), the premaxillse and maxillae both enter the border of the 

 moderate-sized mouth; the preopercular is rudimentary. 



Notacanths. — ^Another deep-sea fish, Notacanihus, differs from the halosaurs in having 

 a very blunt snout, a small mouth and the dorsal fin represented by six separated spines; 

 but Boulenger (1910, pp. 622, 624) showed that a third form, Lipogenys, is intermediate in 

 structure between the Halosauridae and the Notacanthidae. According to Smith Wood- 

 ward (1901, p. 168), the notacanths were represented in the Upper Cretaceous by Pronota- 

 canthus. 



Tate Regan (1909a, p. 82) states that "In skeletal characters Halosaurus and Nota- 

 canthus agree in that the orbito-rostral part of the cranium is elongate, the parietals meet, 

 opisthotics, basisphenoid, alisphenoids and orbitosphenoid are absent, the parasphenoid 

 unites with the sphenotic (postfrontal) in front of the prootic, the posttemporal is simple 

 or ligamentous, . . . ." The otolith (sagitta) of Notacanthus is described by Frost (1926, 

 p. 466) as being quite different from that of Halosaurus in its elliptical contour and unusual 

 thickness; its sulcus resembles that of Halosaurus in length and position but differs in other 

 details. Hence the evidence from the otoliths indicates merely that Notacanthus is more 

 highly specialized than Halosaurus but possibly related to it. 



Dercetids. — This Upper Cretaceous family is also regarded by Smith Woodward (1901, 

 Pt. VIII, pp. 162-189; 1903, pp. 64—74) as apparently related to the Halosauridae and Nota- 

 canthidae. Leptotrachelus as restored by Smith Woodward (1903, p. 69) is an elongate 

 sagittiform fish with a large homocercal tail, an elongate dorsal fin, small abdominal ventrals 

 and a long pointed head. A horizontal row of small scutes extends along the flanks from 

 the head to the tail and there were other rod-like scutes on the flanks. In general it is 

 rather suggestive of the sand-lance {Ammodytes). The latter, however, differs chiefly in 

 having more prolonged dorsal and anal fins, no ventral fins, highly protrusile premaxillae, 

 smaller tail. Thus the resemblances between these two types are very probably con- 

 vergent, especially as Tate Regan has cited good evidence for a quite different allocation 

 of Ammodytes (see below, p. 354). 



As to the interrelationships of the Halosauridae, Notacanthidae and Dercetidae, Smith 

 Woodward states (1901, p. viii) that "Of all the Cretaceous Isospondyli, three families of 

 eel-shaped fishes are the most difficult to understand. They are all characterized by a 

 primitive cranium of the Jurassic type; but they exhibit the new specialization by which 

 the extending premaxilla gradually excludes the maxilla from the upper border of the 

 mouth. Their elongated shape is alone indicative of high specialization but no intermediate 

 forms are yet known to afford a clue to their more normally-shaped ancestors. The 

 Dercetidae are interesting as being the earliest type of fish in which evidence of a distensible 

 stomach has been observed (Woodward, 1901, p. 177). Their fins are less specialized than 

 those of the two families just mentioned; and their trunk is provided with paired longitudinal 

 series of enlarged scutes." 



Tate Regan (1911a, p. 120) holds that: "The Dercetidae are of uncertain relationships, 

 but the orbital and postorbital parts of the skull and the posttemporals show considerable 

 resemblance to Evermanella {Odontostomus) whilst the ethmoid region and jaws are more 

 like those of Alepidosaurus." To this may be added that the general arrangement and 

 form of all the fins in Leptotrachelus is assuredly more favorable to relationship with 

 Alepidosaurus than with any of the halosaurs, notacanths or allied forms. Hence it seems 

 that on such evidence as we have the Dercetidae had better be transferred to the Iniomi. 



