scHucHERT] CLASSIFICATION OF ATREMATA. 119 



developed braiu and infratesopbageal ganglionic swellings. Blood- 

 vascnlar system probably present, with the sinuses developed into vas- 

 cular dilatations at the back of the stomach and elsewhere. Sexes 

 separate. Exclusively inhabitants of the sea. The class is present 

 in the Lower Cambrian, attained maximum development in the Silu- 

 lian and Devonian, and is represented by about 140 living species. 

 During this time, probably upward of G,OU() fossil and recent species 

 have been developed, and these are distributed in 328 genera, grouped 

 in 31 families, 10 superfaniilies, and 4 orders. 



Order ATREMATA Beecher, 1891.i 



Mesokaulia, or Lingulacea (partim) Waagen, 1885. 



Inarticulate Brachiopoda with the pedicle emerging freely between 

 the two valves, the opening being more or less shared by both. Growth 

 taking place mainly around the anterior and lateral margins, never 

 inclosing or surrounding the pedicle. Aperture unmodified. Prodel- 

 tidium attached to dorsal valve. 



Superfamily OBOLACEA Schuchert, ISOG.^ 



Rounded or semicircular and more or less lens-shaped, thick-shelled, 

 primitive Atremata, tixed by a short pedicle throughout life to extra- 

 neous objects. 



1.^ Family PATERINID.E Schuchert, 1893 (emend.).^ 



Obolacea with the dorsal valve semicircular and the ventral sub- 

 circular in outline. Posterior region more or less closed by cardinal 

 areas. 



Iphidea Billings, 1872. 

 Paterina Beecher, 1891. 



Volborthia von Moller, 1873. 



2. Family OBOLID^E King, 1846. 

 Obolinaj Gill, 1871. 



Thick-shelled Obolacea of nearly circular or ovoid outline, biconvex, 

 usually smooth, with rudimentary cardinal areas traversed by shallow 



' Siuce ill tliis classiflcatioii no superoidiiial terms are for the present adopted, it will be well fo give 

 here all such terms used by authors and others which are of lower rank and not readilj' referred as 

 synonyms to their proper places : 



Ancylobracbia Ancylopoda, Helictopoda, Sarcicobrachia Gray, 1848; Lyopomata and Arthropo- 

 luata Owen, 1858; Pleuropygla, Sarcicobranchiona, Sclerobranchiona Bronn, 1862; Articnlata and 

 Inarticulata Huxley, 1864; Clistenterata and Tretenterata King, 1873. 



'Text book of Paleontology, by Zittel and Eastman, 1896, p. 305. Also see page 78 of this bulletin. 



3 The numbers and letters before a family or subfamily term indicate the phyletic relations which 

 these have to one another within a superfamilj*. The phylogeny of the families, however, is more 

 clearly represented in the diagram on PI. I, facing p. TM. 



•Kecent discoveries have shown that Iphidea has no pedicle opening, and should include forms 

 referred to Paterina. Therefore this family is of doubtful value, and is provisionally retained for the 

 reception of genera more primitive in structure than those of the Obolidae. 



