428 Forty-second Report on tee State Museum, 



Faunal Characters of the Lower Helderberg. 



POSITIVE ELEMENTS. 



The Ckustacean element is, for the purposes of a diagnosis, of the 

 first importance. The Trilobitic genera present are Homalonotus, 

 Bronteus, Dalmanites, Phacops, Acidaspis, Lichas, ProUus, Phaethonides 

 and Cyp>haspis. 



The single species of Homalonotus (H. Vanuxemi) possesses the 

 strongly annulated pygidium characterizing the earlier representa- 

 tives of the genus (e. g., H. delphinocephalus of the Niagara, H. major 

 of the Oriskany), and in this respect is in strong contrast to the 

 latest members of the genus, with obsolete pygidial ribs. On 

 the other hand the species presents the smooth unlobed glabella 

 of the prevalent H. De Kayi of the Hamilton, and therein indicates a 

 progress from the Silurian type of Trimerus to the Devonian Dipleura. 



Bronteus Barrandii, and the allied form, B. Pompilius from the Square 

 Lake (Maine) fauna, are species with comparatively small pygidia 

 having a simple, unbifurcated median rib; representing a group pre- 

 vailing in the Lower-Devonian faunas of Europe. The margins of 

 the pygidia are smooth, not spinous as in the subdivision Thysano- 

 peltis, a strictly Devonian group. 



The abundant representatives of Dalmanites are typical members of a 

 wide-spread, lowest Devonian group, namely, that following the type of 

 D. Hausmanni (subgenus Hausmannia, Hall). Strictly conformable to 

 this type are the species D. pleuroptyx and D. micrurus. Also con- 

 formable in general expression and the essential features of simple 

 and complete glabellar lobation, are D. nasutus, D. tridens, D. tri~ 

 dentiferus, which present ornamental extravagances in the pro- 

 longation of the frontal border into a furcate proboscis; also 

 D. dentatus, which possesses a serrate frontal border, indicating its 

 close alliance to D. regalis of the Schoharie grit and to D. pygmceus 

 of the Corniferous limestone, these three species constituting the 

 subgenus Gorycephalus, Hall, as far as its representation is known. 

 D. pleuroptyx, which continues its existence into the Oriskany and Cor- 

 niferous, has a somewhat explanate crenulated frontal border, indicat- 

 ing the inception of the peculiar dentate frontal ornamentation in the 

 Upper Helderberg subgenus Odontocephalus. The remarkable variety of 

 ornamentation in all these trilobites is of itself an important character, 

 irrespective of the genera represented, for it is in Devonian faunas 

 that such features are carried to an extreme. (See Palaeontology of 

 N. Y., Vol. VII, Dalmanites, Bronteus, Geratolichas, Arges, Terataspis, 

 Acidaspis, and Gyphaspis; also a great number of species of these and 

 other genera in the European Devonian.) It is noteworthy that a 



