i 
No. 5.— Notes on the ontogeny of Isotelus gigas Dekay. 
By Percy E. RaymMonp. 
INTRODUCTION. 
A partial description of the ontogeny of three of our common 
asaphids, [sotelus gigas, I. maximus, and Basilicus barrandi, is given 
on the following pages. The study is based on many hundreds of 
good specimens, but, as always in palaeontologic work, the material 
leaves something to be desired, and further specimens will add ma- 
terially to our knowledge. The important protaspis stages are still 
missing. 
The outstanding result of the study is the discovery that an Isotelus 
gigas, when 3 to 5 mm. long, has almost exactly the same form as an 
adult specimen of Basilicus barrandi, thus providing an excellent 
example of recapitulation, for the beginning of the range of Bastlicus 
antedates that of Isotelus. Another interesting fact is that while 
Tsotelus gigas is the most specialized species of the genus, it is one of 
the first to appear, and apparently one of the first to die out, while 
the ones which survived to the end of the Ordovician were the more 
primitive forms, [sotelus maximus and I. towensis. 
Two of the three species of the Chazy, Isotelus harrist and I. platy- 
margmatus, do not seem to have had any influence on the more 
persistent and widespread species which belong to the interior faunas. 
Tsotelus harrisi (Ann. Carnegie mus., 1905, 3, p. 343) has a broad flat- 
tened cranidium, and is not allied to any other species except the 
Russian I. stacyi, to which Schmidt compared it. Tsotelus platu- 
margimatus (Ann. Carnegie mus., 1910, 7, p. 66) has a very wide de- 
pressed border on both shields, and is quite unlike any of the later 
Species. The third species, I. arenicola (Ottawa naturalist, 1910, 24, 
p. 130), is more like I. cowensis or I. gigas, and may. have given rise to 
one or both of those species. While it has the specialized long pygi- 
dium, the axial lobe is narrow and the genal spines are retained at 
_ Maturity. 
