l66 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



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 Apodidae 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. 



Class CRUSTACEA 



Sub-Class BRANCHIOPODA 



Order ANOSTRACA Caiman* 



OPABINID^, new family 



Carapace absent ; paired eyes pedunculate ; antennae 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 Thamnocephalinse, Abdomen a simple plate, 



with two caudal, unsegmented furcal rami on the female. 



The Opabinidae differ from .the most nearly allied family, Tham- 

 nocephalinas Packard, in having a simple plate-like unsegmented 

 abdomen. 



OPABINIA, new genus 

 The generic and specific descriptions are united under the descrip- 

 tion of the species. 



^The Apodidae. Nature Series, London, 1892, p. 9. 



' Walcott, Monogr. U. S. Geol. Survey, Vol. 8, 1884, p. 262. 



° Idem, p. 261. 



^As defined in Lankester's Treatise on Zoology, London, 1909, Pt. 7, p. 53. 



