No. 5. — Notes on the ontogeny of Isotelus gigas Dekay. 

 By Percy E. Raymond. 

 Introduction. 



A partial description of the ontogeny of three of our common 

 asaphids, Isotelus 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 Basilicus 

 antedates that of Isotelus. Another interesting fact is that while 

 Isotelus 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, Isotelus maximus and /. iowensis. 



Two of the three species of the Chazy, Isotelus harrisi and /. platy- 

 marginatus, do not seem to have had any influence on the more 

 persistent and widespread species which belong to the interior faunas. 

 Isotelus 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 /. stacyi, to which Schmidt compared it. Isotelus platy- 

 marginatus (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, /. arenicola (Ottawa naturalist, 1910, 24, 

 p. 130), is more like I. iowensis or /. 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. 



