Dr. F. R. Coivper Reed — The genus Homalonotus. 827 



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 Pygidium triangular, subconical, obtusely pointed behind, with 

 trilobalion obsolete or obsolescent and segmentation very faintly 

 marked. 



Type. — H. Dekayi, Green. 



Eange. — Middle Devonian (Hamilton Formation). 



Distribution. — North America. 



Kemakks. — Tliis subgenus seems confined to North America and 

 to be the latest representative of the genus Momalotiotus. It is 

 extremely doubtful if the Harz species S. Schusteri, Roem.,' is 

 rightly referred to Bipleiira by Kayser.''^ The characteristic features 

 of Dipleura are the large pre-sutural prora (recalling that of 

 Btir^neisterella), the straight transverse commissure of the facial 

 sutures (somewhat as in Bigonus), the direct continuation of the 

 facial into the epistomal sutures, and the obsolete trilobalion and 

 nearly obsolete segmentation of the pygidium. The last-mentioned 

 character is, however, also found in H. Icevicauda, Quenst., attributed 

 provisionally to the preceding subgenus, Faraliomalonotus. 



CONCITTSION. 



"With regard to the stratigraphical distribution of the above- 

 described ten subgenera of the genus Homalonotus, we note that the 

 first three, Eohomalo7iotus , Calymenella, and Brongniartella, are 

 restricted to the Ordovician, the first one being the earliest; Trimenis 

 and Kcenigia occur only in the Silurian, and all the rest, Burmeisteria, 

 Bigonus, Burmeisterella, Parahomalonotus, and Bipleura, are found in 

 the Devonian. It is clear, therefore, that the climax of development 

 was reached in the Devonian, and it is remarkable that the genus did 

 not survive this period. 



The phylogeny of the genus is imperfectly known. We have seen 

 that it may be linked with Calymene by means of SynJiomalo7iotus, 

 though it must have diverged at an early period, or more probably 

 have originated from a common stock. Within the limits of th.e 

 genus the relationships of the different subgenera are difficult to 

 trace. There is a considerable morphological gap between the 

 Ordovician and Silurian groups, and transitional forms are at present 

 unknown. The Devonian subgenera fall into two main groups, one 

 of which, comprising Burmeisteria and Bigojius, suggests a connexion 

 with the Silurian subgenera, but on the other hand Parahomalonotus 

 suggests reversion to the earlier types. Biirmeisterella in some 

 respects also points back to Ordovician forms, but it is undoubtedly 

 highly specialized. Bipleura may be a modification of the Bigonus 

 type, and is the latest representative of the genus in any part of 

 the world. 



I am much indebted to the authorities of the Sedgwick Museum, 

 Cambridge, the British Museum, the Jermyn Street Museum, the 

 Siirewsbury Museum, and the Ludlow Museum for the opportunities 

 afforded me of examining specimens in their collections. 



' Eoemer, Beitr. z. geol. Kennt. nordw. Harzgeb., iii, t. iii, fig. 20, 1855. 

 ^ Kayser in appendix to Koch, op. cit. , p. 76. 



