COMPAR/VTIVE STUDY OP SCOMBKOID FKHES. 353 



Tbc olüictüiy oij^uus ai"o a jxiir of grooves in front of tlic cy&s. Each 

 RTOovo or sac cominunicatos witb tbo exterior by a pair of pores, nostrils. 

 The anteiior nosfciil is generally small, while the jX)st«rior is more or less 

 elongated, oblong in the Cybiidae, and quite a sUt in the Scombridae and 

 Pleoostei. Beneath tlio antei'ior nostril, tliei-o is a group of olfactory leaves, 

 about 30 in number, aiTanged radially, in the form of a rosette. In the 

 Scombridae two nostrils ai'o situated rather near each other, and the upper 

 wall of the olfactory cavity is imiformly thin. There is a deep gi'oovo in the 

 floor of the cavity before the ethmoid, and just behind the olfactory rosette. 

 Tlie cavity extends behind the groove. The inner opening of the posterior 

 nostril Ues above the groove or before it. 



In the C3'büdae the olfactory cavity is rather small, and the two nosti-ils 

 are close t«:;ether, the whole cavity is neai-ly filled with the rosette of the 

 olfactory leaves. In this family the posteiior nostril Hes more or less behind 

 the i-osette. The doi-sal wall of the cavity is thin, but the border of the inner 

 orifice of the posterior nostril is generally raised. Moreover the dorsal wall is 

 thickened in Sarda. Generally the cavity does not extend behind the posterior 

 nostril, and thei'e is a fleshy dam behind the rosette and below the posterior 

 nostril. 



In the Thunnidae there is a space behind the rosette, thus the two 

 nostrils are much sepai-ated. The anterior nosti-il in veiy minute. The rosette 

 of the olfactory leaves is liigh and occupies the whole height of the cavity. 

 The dorsal wall of the cavity is very thick. 



In the Katsuwonidae the two nosti'ils ai'e close together", and the cavity 

 is much more depressed than in the Thunnidae. Tlie i^assage of the anterior 

 nostril is almost pei^pendicular to the smiace of the head, while that of the 

 posterior is more or less turned obUquely. The former has the uniform calibre, 

 but the latter is wide at the inner orifice, and becomes gi-adiuüly naiTow towards 

 the outer orifice. Between these nosfa-ils there is a narrow groove on the roof 

 of the olfactory cavity. 



ABDOMINAL CAVITY, 



In the Scombiidae the height of the abdominal ca%ity is more than half 

 the height of the body, and the cavity Ues just beneath the vertebral column ; 



