112 IOWA GEOLOGICAL SURVEY 



also in the Carboniferous limestone of the Government of 

 Moscow, Russia. 



Mention may also be made in this connection of the forms 

 of pavement dentition known as Helodus gibberulus Agassiz, 

 from the Chemung and Waverly rocks of several localities in 

 Pennsylvania, and the newly described H. comptus, from the 

 Meadville Upper Shale, in the vicinity of Meadville, western 

 Pennsylvania. The latter exhibits characters transitional be- 

 tween Helodus and Orodus and is delicately plicated. 



Order ACAMHODII. 



Endoskeleton well calcified, with dermal and membrane calci- 

 fications in the regions of the skull and pectoral arch. Pterygo- 

 quadrate arcade movably articulated with the cranium (hyo- 

 stylic). Orbit surrounded with a ring of thin plates of dentine. 

 Teeth, when present, firmly fixed to the calcified sheathing plates 

 of the pterygoquadrate and mandibular cartilages. Endoskel- 

 etal cartilages of all the fins much reduced, and the dermal ex- 

 pansion almost or completely destitute of rays ; each of the fins 

 except the caudal with a robust anterior spine implanted in the 

 flesh. Pelvic fins of male without claspers. Dermal armature 

 of trunk consisting of small, closely arranged quadrate granules, 

 which also extend over the greater portion of the fins; lateral 

 line passing between two series of granules. 



This large and characteristic group of Palaeozoic sharks is re- 

 markable for developing membrane calcifications in the region of 

 the skull and pectoral arch. The endo skeletal cartilage is also 

 more or less permeated with granular calcifications, though the 

 supports for the fins are never preserved. Two families of Acan- 

 thodian fishes are commonly recognized, one distinguished by 

 the presence of a single, and the other by two dorsal fins. Both 

 families are represented in the American Palaeozoic, though their 

 remains are infrequent, and as a rule imperfectly preserved. In 

 Europe the Acanthodidae range from the Lower Devonian to the 

 Permian inclusive, but the Diplacanthidae are limited to the up- 

 permost Silurian and Lower Devonian. It has long been con- 

 sidered probable that the detached fin-spines known as Mach- 

 aeracanthus belong to members of this order, and thanks to the 



