scHuciiKHT] CLASSIFICATION OF ATREMATA. 119 



developed brain and intra(i'S()])liai;eal gaiij;Iii)iiic swellings. Hlood- 

 vascular system probably i)resent, with tlie sinuses developed into vas- 

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

 sejtarate. Exclusively inhabitants of the sea. The class is [»resent 

 in the Lower Cambrian, attained niaxinium development m the Silu- 

 lian and Devonian, and is represented by about l-tO livinj;- species. 

 Durinji' this time, probably uj)ward of (),0{)() fossil and recent species 

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

 in ol families, 10 su[)erfamilies, and 1 orders. 



Order ATREMATA Beecher, 18<J1.' 



Mesokaiilia, or Liuj^iilticea (partiui) Waagen, 1885. 



Inarticulate lirachiopoda with the pedicle emerging freely between 

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

 taking place nniinly around the anterior and lateral margins, never 

 inclosing or surrounding the pedicle. Aperture nnmoditied. Prodel- 

 tidiuni attached to dorsal valve. 



iSuperfamily OBOLACEA Schuchert, 1890.^ 



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

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

 neous objects. 



1.' l^amily 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. 

 Ipiiidea Billings, 1872. 



Pateiiua Beecher, 1891. 



Volborthia von Moller, 1873. 



2. Family OBOLID^ King, 1846. 



Obolina^ Gill, 1871. 



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

 usually smooth, with rudimentary cardinal areas traversed by shallow 



' Since in tliis classification no siiperordinal terms are for the present adopted, it will bo well to give 

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

 synonyms to their i)roper places: 



Ancylobracliia Ancyloijoda, Helictopoda. Sarcicobracliia Gray, 1848; Lyopomata and Arthropo- 

 inata Owen, 1858; Pleuropygin, Sarcicobranehioiia, Sclurobranchiona Bronn, 1862; Articulata aud 

 Iniirticulata Iluxley, 1864; Clistenterata and Tretcntorata KiuK, 1873. 



'Text book of Paleontology, by Zittcl and Eastman, 1896, j). 305. Also see page 78 of this bulletin. 



^The numbers and letters before a family or subfamily term indicate the i)hyletic relations which 

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

 clearly represented iu the diagram on PI. I, facing p. ]li4. 



'Recent 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 Obolidie. 



