Page:Walcott Cambrian Geology and Paleontology II.djvu/248

 varied Middle Cambrian fauna, proves that in that early time they were capable of flourishing in the midst of active and powerful enemies. This was owing undoubtedly to their great power of reproduction and active movements.

Bernard attributes the preservation of the Apodidæ in geologic time to the isolated manner of life of the animals. This may be true since Carboniferous time, but I doubt if it was so during the long, early Paleozoic ages. The evidence for the existence of a land surface since early Carboniferous time with continuing streams or ponds is found in the presence in Lower Carboniferous strata of fresh-water shells that were undoubtedly the ancestors of the living fresh-water genera Physa and Ampullaria. It may be that the descendants of the Cambrian Branchiopoda became adapted to fresh-water conditions in Devonian time after the disappearance of the large group of merostomes that reached its greatest development and almost disappeared in Silurian time.

That the smaller and more delicate forms of the Branchiopoda have not been found in Ordovician, Silurian, and later rocks is no proof that they did not exist side by side with the thick shell-covered crustaceans that have only left traces here and there in the sediments.

Carapace absent; paired eyes pedunculate; antennæ unknown, frontal appendage (proboscis) flexible, prehensile in male, bifid in female. Trunk limbs 16 pairs, the terminal joints of the feet broad and spatulate as in the Thamnocephalinæ, Abdomen a simple plate, with two caudal, unsegmented furcal rami on the female.

The Opabinidæ differ from the most nearly allied family, Thamnocephalinæ Packard, in having a simple plate-like unsegmented abdomen.

The generic and specific descriptions are united under the description of the species.