CLUPEA ALOSA. 259 



much less in males and in half-grown females. In the young state the body- 

 is four and a half times as long as high, and the height is less than the length 

 of the head, which is one-fourth of the length. The head is conspicuously 

 longer than high. The breadth between the opercula is less than half a head 

 length, but in the young fish the thickness is one-quarter of the length of the 

 head. In old females the eye is one-sixth as long as the head, and the inter- 

 orbital space has widened from two-thirds of an orbital diameter to twice the 

 diameter of the eye. 



The eye is covered both anteriorly and posteriorly with a fold of skin, 

 which forms a transj^arent eyelid over the middle of the eye, opening in the 

 centre with a vertical slit. This skin includes cartilaginous plates, which 

 have a half-moon shape, so that the pupil is not completely covered, and the 

 anterior plate can pass over the posterior plate. One eyelid extends under the 

 nai'cs, and the other to the opercular bone. These eyelids have been compared 

 to the fold seen at the hinder margin of the orbit in Salmon. 



The nares are divided by a small membranous band, so that the posterior 

 narine is the longer and larger. The mouth is terminal, rather wide, and very 

 oblique. A notch divides the small pre-maxillary bones from each other. The 

 jaws are equal. The cleft of the mouth extends back under the orbits. The max- 

 illary bone resembles that of the Grayling, except in being composed of three 

 pieces, since it reaches as far back as the hinder border of the orbit. Both the 

 maxillary and pre-maxillary are furnished at their margin with extremely fine- 

 pointed short teeth which are easily lost, and disappear early. The sub-orbital 

 ring consists of six bones, is broad, and the maxillary bone extends under its 

 wide anterior portion. The mandible is overlapped by the wide upper jaw ; it 

 is toothless, but teeth exist in the young, and are soon shed. The gill-aper- 

 ture reaches higher than the upper border of the orbit, and the opening extends 

 far into the throat. Of the eight branchiostegal rays, the last two end in a 

 bony plate which unites with the inter-operculum and sub-operculum to form a 

 peculiar cavity in front of the base of the pectoral fin. The pre-operculum is the 

 largest opercular element, and extends higher than the sub-operculum. There 

 are thirty-three thoracic vertebrae and twenty-five in the caudal region. The 

 number of bones of the nature of ribs attached to the vertebral column is 

 unusually large. 



All the fins are relatively small. The dorsal fin is longer than high. 

 It begins in front of the middle of the body ; its last ray is more pro- 

 longed than those which immediately precede it. When the fin is laid back 

 it is partly hidden in a furrow formed by the scales, which are raised on each 

 side of its base. The vent is situate, three times the length of the head, behind 

 the snout. The anal fin is immediately behind it; its base is as long as the 



