No. 2.] LATERAL LINE IN EXTINCT AMPHIBIA. 519 
fectly distinct from that found in Necturus, although the same 
general plan is observable in all. The Necturus is, I believe, 
a more generalized type of amphibian so far as the lateral 
line is concerned than any of the other modern forms. This 
preservation of the lateral line system so perfect is due, with- 
out doubt, to the constant water habitat of the animal. Kings- 
bury (12) has expressed it as his opinion that Necturus is a 
primitive form and bases his conclusions on other grounds 
than that of the lateral line system. In very few of the modern 
Amphibia is the median line present as far back as the tip of 
the tail and in none, so far as I can learn, are the median and 
dorsal lines both present on the tail except in the Necturus. 
Other forms have lost the line from the tail, and this but 
gives expression to the general law that structures present over 
the entire body are lost first in the posterior region. This 
finds another expression in the loss of stripes by the zebras 
where in the quagga the hind part of the animal is destitute 
of stripes. In Ambystoma, for instance, the median lateral 
line is not present on the tail at all and the dorsal line is but 
imperfectly developed (13). The close similarity of the 
arrangement of the sense organs in the two forms, Necturus 
and Micrerpeton, may be of genetic significance with regard 
to the former group. 
“The interval of time which has elapsed from the age in 
which Micrerpeton lived to the present is reckoned by many 
millions of years. But since the lateral line organs are of 
fundamental significance and since they are subject to compar- 
atively little variation, this system of sense organs in the 
Amphibia may have persisted through the ages unchanged as 
we know it has done in some of the fishes (14). The ances- 
tors of the modern Caudata and Salientia must have originated 
somewhere in the Carboniferous or earlier ages. Among all 
of the extinct Amphibia there are none which could have 
given rise to the modern forms save the Branchiosauria. The 
Microsauria are too highly specialized when we first know 
them in the Carboniferous, and they are already tending 
toward the reptilian type, to some groups of which they prob- 
