324 



MEMOIRS OF THE CARNEGIE MUSEUM. 



contains within itself certain potentialities of continuous variation. It is charged, 

 so to speak, with a complex of latent characteristics. One set of these is that which 

 terminates in an elongated tubiform snout, the other in the development of a 

 peculiar kind of dermal armor, including a long and slender dorsal fin-spine. In 

 the tube-snout evolutionary series the gradual elaboration of these two sets of 

 characters, which may be supposed to be resident potentially in Gasterosteus, goes 

 hand in hand; and thus we find. that Centriscus has both a tubular snout and is 

 provided with dermal armor and a well-developed dorsal fin-spine. But in the 

 rostrate series one of these sets of characters is suppressed, no tubular snout being 

 developed. The second group of characters which was potentially present in 

 Gasterosteus is developed in precisely the same fashion as in the Centriscoids, with 

 the result that in Rhamphosus we find a body-armor paralleling that in Centriscus, 

 and a remarkably similar dorsal fin-spine. The divergence in forms with reference 

 to the splitting up of the original complex of characters might be illustrated by the 

 following scheme : 



Fistulariids 



t 

 Aulostomids 



I 

 Urosphen 



I 

 Aulorhynchus 



Amphisile 



T 



Centriscus 



Rham 



)hosus 



Gasterosteus 



Family Rhamphosid^ Gill {emend.). 

 Solenichthyes with Gasterosteus-like form of body, the anterior vertebrse 

 discrete and not elongated, about twenty-two (eight abdominal and eighteen caudal) 

 vertebrse in all ; dermal plates on the nape and shoulder-region only; a single, elongate 

 dorsal spine arising from the hinder end of the nuchal armature. Mouth small, 

 and placed as in Gasterosteus, but the upper portion of the head produced in an 

 elongate rostrum. Ventral fins subthoracic, the second dorsal and anal remote and 

 opposite. 



2. Rhamphosus rastrum (Volta). 



(PL XLIV, Figs. 1-3). 

 1796. Uranoscopus rastrum G. S. Volta, Ittiolit. Veronese, p. 22, pi. V, fig. 4. 

 1796. Centriscus G. S. Volta, ibid., pi. LXXV, fig. 1 (errore). 



