Isospondyli 53 
figured. Numerous herring, referred to Clupea, but belonging 
rather to Pomolobus, or other non-Arctic genera, have been 
described from the Eocene and later rocks. 
Several American fossil herring-like fishes, of the genus 
 Leptosomus, as Leptosomus percrassus, are found in the Cretaceous 
of South Dakota 
Fossil species doubtfully referred to Dorosoma, but perhaps 
allied rather to the thread-herring (Opisthonema), being herrings 
with a prolonged dorsal ray, are recorded from the early Ter- 
tiary of Europe. Among these is Opisthonema doljeanum from 
Austria. 
The Dorosomatide.— The gizzard-shad, Dorosomatide, are 
closely related to the Clupeide, differing in the small contracted 
toothless mouth and reduced maxillary. The species are deep- 
bodied, shad-like fishes of the rivers and estuaries of eastern 
America and eastern Asia. They feed on mud, and the stomach 
is thickened and muscular like that of a fowl. As the stomach 
has the size and form of a hickory-nut, the common American 
Fig. 42.—Hickory-shad, Dorosoma cepedianum (Le Sueur). Potomac River. 
species is often called hickory-shad. The gizzard-shad are all 
very poor food-fish, bony and little valued, the flesh full of 
small bones. The belly is always serrated. In three of the 
four genera of Dorosomatide the last dorsal ray is much produced 
and whip-like. The long and slender gill-rakers serve as strainers 
for the mud in which these fishes find their vegetable and ani- 
mal food. Dorosoma cepedianum, the common hickory-shad or 
