550 Wisconsin Academy of Sciences, Arts, and Letters. 
The genus TJiamnopJiis differs from Zamenis in that the inter¬ 
costal arteries plunge into the mid-line of the dorsal parietes 
and are not paired, while in Zamenis they enter to the left or 
right and are frequently paired (Atwood, T6). In the Boidae 
(Beddard, ’04, ’06, ’08, ’09), the condition of the intercostals 
varies but they do not enter the mid-line of the dorsal parietes 
and are usually paired. In their primitive condition it is prob¬ 
able that a pair entered each intercostal space and in the special¬ 
ized forms reduction has taken place. If this assumption is true 
TliamnopMs is rather highly specialized in this respect. 
In regard to the right anterior azygous vein Thamnophis and 
Tropidonotus (O’Donoghue, ’12) stand rather low in the scale 
of specialization. 
The writer (’16) has mentioned finding many trematodes and 
nematodes in the viscera of Zamenis constrictor. Very few of 
these were found in TTiamnopkis. This comparative immunity 
may be a factor in the relative abundance of the garter-snakes. 
The writer (’16) has previously commented upon the great 
individual variability of snakes and the advisability of examin¬ 
ing many specimens before making statements. Much of the 
work on the anatomy of the Serpentes is based on the examina¬ 
tion of only one or two specimens. 
Thompson (’14) gives some notes on the visceral anatomy 
of one specimen of T. ordinoides. His description does not in¬ 
dicate that it differs specifically from the specimens of T. sirt- 
alis described in this paper. He states that the maximum num¬ 
ber of dorsal scale rows in this species varies from twenty-one to 
seventeen. This fact alone indicates that it is extremely variable. 
Kuthven (’08) records cases where certain species of TJiam- 
nopJiis gave birth to young bearing the characteristic marks of 
other species. Many specimens of Thamnophis have been dif¬ 
ferently classified by different authorities. The facts set forth 
in this paper show that there is great variation in different in¬ 
dividuals. In consideration of the above facts it seems likely 
that the individual variability of Thamnophis has been too 
largely taken to represent specific variability. The writer de¬ 
sires to express his admiration for the excellent efforts of Ruth- 
ven (’08) to synthesize this genus. 
