16 U.S. NATIONAL MUSEUM BULLETIN 282 



The anterior position of the first nasal pore (Nl) is unique in Pylo- 

 dictis, but its anterior position is approached in Satan. The number of 

 sensory canal pores anterior to the outer mental barbel (see p. 12) appears 

 to be of some evolutionary significance. Another kind of pore, "the 

 venom pore," located immediately below the posterior process of the 

 cleithrum is large in most species of Noturus and in young of Ictalurus 

 and Pylodictis; in Prietella and large individuals of the previous genera 

 it is greatly restricted in size, but it appears to be absent in Satan and 

 Trogloglanis. 



Ictalurus group. — The species in the genus Ictalurus are a cluster 

 of divergent forms having the following characters in common: ten 

 preoperculomandibular pores, eight pelvic rays, and an extremely 

 shortened, high adipose fin which is free from the back posteriorly and 

 remote from the caudal fin. The bullheads, Ameiurus [=Amiurus] are 

 regarded as members of the genus Ictalurus because there is gradation 

 without a significant break from them through several intermediates 

 to the most divergent Ictalurus such as /. punctatus. In young Ictalurus 

 the body may be light grayish blue with black vertical fin margins, 

 mottled, spotted with light or with dark, or brown or blackish above 

 and lighter below. With age and development of breeding characteris- 

 tics these fish assume a color that is almost uniformly dark, especially 

 on the dorsal and lateral surfaces. In some forms the skull is greatly 

 arched, especially in the young; in a few it is flattened or depressed, 

 and in others it is variously intermediate in shape. With the develop- 

 ment of the breeding characters, the skulls broaden and become more 

 depressed. Thus the high arched heads are transformed into shallowly 

 rounded structures and the flattened heads are even broader and more 

 depressed by the change in bone relationships as well as the prolifera- 

 tion of muscle masses laterally and occipitally. Consequently, a breed- 

 ing adult may show little resemblance to its young but instead may 

 resemble the young of another species. 



The shape of the caudal fin also varies or grades from rounded 

 posteriorly through emarginate, notched, shallowly forked, and deeply 

 forked among the species. The depth of the fork is extensively re- 

 duced with age and breeding condition so that species with deeply 

 forked tails as young have only a deeply notched or very shallowly 

 forked tail when older, again often resembling the young of another 

 species. Only two of the species, Ictalurus furcatus and /. punctatus, 

 have a continuous bony bridge from the supraoccipital to a small 

 bone extending forward from the base of the dorsal fin. The supra- 

 occipital ("occipital") process in others is variously developed, from 

 an elongated shelf that is widely separated from the dorsal fin and 

 lacks connecting elements, to a near union with the process from the 

 dorsal fin base. Two of the species, Ictalurus furcatus and /. balsanus, 



