286 bulletin: museum of comparative zoology. 



the Middle Ordovician of Minnesota and Pennsylvania. The essen- 

 tial features of the trilobites for which this name has been used are 

 the isoteliform glabella and sutures, but asaphiform lack of concave 

 borders on the shields. Since my attention was first directed to this 

 combination of characteristics wider experience has shown that they 

 form a large group in the Middle and Upper Ordovician and that 

 while similar to the Russian Onchometopus they are probably not 

 congeneric with it. The two genera form one more example of that 

 "Parallelism among the Asaphidae" to which I have already called 

 attention (Trans. Royal soc. Canada, 1912, 5, sect. 4, p. HI). De- 

 tailed study with large numbers of specimens indicates that Oncho- 

 metopus is a derivative of Asaphus, while Homotelus sprang from 

 Isotelus, not once merely, but probably several times. Homotelus 

 differs from Isotelus chiefly in lacking the concave borders on the 

 shields. Often specimens are found in which concave borders are 

 feebly or sometimes even well developed, indicating that this char- 

 acteristic is one of suppression and showing readily how a Homotelus 

 could have been evolved time after time from various species of 

 Isotelus. Homotelus cannot then be regarded in a strict sense as a 

 good genus, but is a convenient term for a number of species showing 

 similar characteristics. 



At the time of our first use of Onchometopus, Mr. Narraway and I 

 pointed out that the American species which we referred to the genus 

 did not have the peculiar hooked doublure which Schmidt consid- 

 ered the most important feature. In 1914 it was my privilege, aided 

 by the Shaler ^Memorial fund, to collect Onchometopus from the 

 typical localities south of Lake Ladoga, and direct comparison of 

 specimens is now possible. Cephala of American and Russian forms 

 are exceedingly alike in smoothness of glabella, position of eyes, 

 course of facial sutures, and shape of fixed and free cheeks. Doublures 

 are strikingly different not only in the hooked and furrowed character 

 of the Russian form, but also in its narrowness. Greater differences 

 are seen in the thorax, where Onchometopus shows the high narrow 

 rings of an Asaphus while the Homotelus has the broader and flat- 

 tened rings of the Isotelus. The pygidia again are similar but that 

 of Onchometopus is generally shorter and more nearly semicircular. 



As the type of Homotelus I am selecting a species from the Eden 

 of the region of Cincinnati, chosen because of the excellent material 

 available, and named Homotelus ulrichi for Dr. E. O. Ulrich of the U. S. 

 Geological Survey. 



