874 
P. nodosus, 
P. jeffersonius, 
P. latiauratus, 
P. pallium, 
P. madisonius, 
P. gibbus, 
P. eboreus, 
P. operculatus, 
P. hericeus, 
P. varius, 
THE AMERICAN NATURALIST. 
Southeastern North America 
Miocene fossils, S. E. North America . 
var. monotimeris, California 
Pacific Islands 
precursor of P. jefersnius, fossil 
var. amplicostatus 
var. borealis, recent . 
from New England, aan 
* South Shore, L. I., irradians 
* Cold Spring Harbor, L. I., irradians 
. irradians, recent 
a « fossil, late Pisces 
. dislocatus, fossil, late Pliocene 
: recent 
precursor of P. gibbus (= 
Miocene and Pliocene . 
British 
Vancouver . 
Naples 
irradians) 
[Vor. XXXIV. 
RIDGES. 
7-10 
GROOVES. 
I2 
16 
17 
18 
19 
24 
From this table it appears that P. ( gibbus) irradians is inter- 
mediate in the number of its ribs between the extremes. 
The 
question arises whether in phylogeny the number of rays has 
been increasing or decreasing to produce P. irradians. 
the data given and on the assumption that P. eboreus is the 
ancestor of P. irradians we have the series : 
P. eboreus, Miocene and Pliocene 
P. irradians, late Pliocene 
P. irradians, recent 
From 
22 ribs. 
. 22-19 ribs. 
19 ribs. 
This seems to indicate that there has been a tendency for 
the number of ribs slightly to decline. 
On the other hand, the 
fossil P. gibbus, var. dislocatus (the southern form of irradians), 
shows an increase from eighteen to twenty ribs in passing from 
the late Pliocene to the present. 
species, such as varius, twenty-four rays; 
Also, the more specialized 
striatus, fifty-one 
rays; miniaceus (South Africa) thirty to forty rays ; and 
islandicus (east coast, United States), thirty-five to fifty rays, — 
