o20 Dr. F. R. Cowper Reed — The genus HoTnalonotus. 



the facial sutures unite in front on the upper surface of the head- 

 shield, although frequently very close to the margin, whereas in 

 Calymene they cut the anterior edge of the head-shield at some 

 distance apart and are connected together on the lower surface. 

 In Uomalonotus, therefore, the median portion of the front margin 

 of the liead-shield is formed by the epistome, sometimes expanded 

 and recurved so as to form a broad pre-sutural prora, whereas in 

 Calymene the same part of the head-shield is composed of the pre- 

 glabellar post-sutural area, and the epistome is wholly confined to 

 the lower surface of the head-shield, not appearing at all on the 

 upper surface. The obsolescence, more or less complete, of the 

 glabellar lobes and the frequent loss of trilobation in the thorax and 

 pygidium of Somalonotus are secondary characters of degeneracy, 

 and can hardly be regarded of primary morphological importance 

 in comparing the two genera. The correspondence in the number 

 of segments in the thorax should not have too much stress laid upon 

 it, for in the fairly homogeneous genus of Illmius, which shows 

 modifications in many respects parallel to Homalonotus, the number 

 of segments varies from eight to ten. 



The earlier species of Homalonotus, such as those from the Gres de 

 May, show a remarkable resemblance in the characters of the 

 pygidium to Calymene, and in some species (e.g. JI. hiserrattis) even 

 the bifurcation of the tips of the pleurae near the margin is indicated. 

 We must, however, remember that even in these early representa- 

 tives of Somalonotus all the special characters of the head-shield and 

 glabella are fully developed. The more distinct trilobation of the 

 body and pygidium is an additional feature in these Grres de May 

 species, and they undoubtedly come nearest to Calymene. 



It is worthy of remark that the genus Calymene itself did not tend 

 to differentiate into subgenei'a, its general character remaining 

 •extraordinarily constant during its whole stratigraphical range, 

 whereas Homalonotun is not nearly so homogeneous an assemblage 

 of species, considerable variation having taken place along more or 

 less distinct lines of development. 



We may note a somewhat remarkable parallelism in the generic 

 life-history of Somalonotus sens. ext. and Asaphns sens, ext., though 

 in the latter case the development took place more rapidly and 

 simultaneously, being practically within the confines of the 

 Ordovician, but in more or less distinct biogeographical areas. 

 A rounded semicircular or transverse head-shield goes with a 

 rounded semicircular or transverse pygidium with an entire margin. 

 A pointed and elongated head-shield accompanies a pointed and 

 elongated pygidium, and the number of segments in the latter 

 similarly increases. A somewhat parallel case exists in the genera 

 Phacops and Bahnanites. 



In Asaphus we may also remark that the facial sutures, which, as 

 in Somalonotus,. uinte on the upper surface, may form a regular 

 curve or meet in a pointed arch or ogive ; they may also lie close to 

 the margin or well inside it, or may even cut the front edge and be 

 connected below it. But there is no pair of epistonial sutures 

 in Asaphns, and the facial sutures cut the hind margin of the 



