Relative Growth in Polyodon 
David H. Thompson 
Rostrum.—The spoonbiil cat, Polyodon spathula 
(Walbaum), has a@ broad, thin, paddle-shaped rostrum of cartilage 
enclosed in a loose network of bony splints. This great rostrum 
with its many sensory endings serves for the detection of plank- 
ton and other small organisms on which this fish feeds. The 
mouth is unusually large and the gill arches are furnished with 
numerous long gill rakers which strain out the food organisms as 
the fish swims about with its mouth open, It swims unceasingly 
with a monotonous rhythm and swings the rostrum from side to side 
in a wide arc. It has been supposed that these movements of the 
rostrum are of use in beating small animal life from vegetation in 
weedy lakes, but there is no evidence of any active digging. 
Distribution.—Polyodon inhabits the Mississippi River, 
its connecting bottomland lakes and bayous, and the lower courses 
of its larger tributaries, including the Ohio, the Missouri, and 
the Illinois. It is a ganoid and resembles the sturgeons more 
than it does other native fishes, It has only one living near 
relative, Psephurus gladius, a large fish found in the Yangtze 
River in China. A fossil form, Crossopholis magnicaudatus, has 
been described by Cope (1883, 1885, I886) from the Eocene Green 
River shales in Wyoming. Fossils of two specimens about a meter 
in length show snouts shorter than those of the two living forms 
(Dean, 1895). 
Measurements.—In May, 1932 we collected seven larval 
specimens Of Polyodon (Thompson, 1933) from which the length of 
rostrum and of body were measured. Extensive series of such mea- 
surements are given by Stockard (1907) and Danforth (1911). Bar- 
bour (1911) figured three specimens which have been measured, and 
Doctor N. Borodin has kindly furnished me measurements of three 
very small specimens in the Museum of Comparative Zoology at 
Harvard. Nichols (1916) published the measurements of a very 
large individual, These have been supplemented by measurements 
of other specimens in the collections of the Illinois Natural 
History Survey. Since these data form an unusually complete series, 
they show in some detail the relation between the length of rostrum 
and the length of body. This comparison of relative growth is of 
especial interest, since the rostrum is relatively very small in 
larvae, relatively greatest in specimens about £50 millimeters 
long, and becomes relatively shorter in larger individuals. The 
data are especially significant since the range of sizes is very 
great, the largest specimen being 127 times as long as the smallest 
specimen. Throughout all these sizes the spoonbill is free living, 
has no abrupt metamorphosis, inhabits the same waters, and feeds 
on the same food. 
The following tabulation shows these measurements ar- 
ranged in order of increasing total lengths. The rostrum and body 
