GENERA OF THE SUBORDER ORTHOIDEA 



65 



Genoholotvpe. — TerebratuUtes hljoratus Schlo- 

 theim 1820, Petref., p. 265. 



Description. Exterior. — Shells spirifcroid, nn- 

 terior margin truncate, emarginatc, or lobate; hinge- 

 line straight, commonly mucronate; cardinal angles 

 acute or obtuse ; lateral profile strongly biconvex, com- 

 monly globular; anterior commissure strongly unipli- 

 cate; umbos inflated, palintropes very broadly tri- 

 angular, curved, subequal, beaks incurved, ventral area 

 vertically striate, ventral beak commonly resorbcd by 

 the pedicle ; delthyrium and notothyrium open ; orna- 

 mentation costate, whole surface covered by minute 

 granules. Test fibrous, impunctate. 



Ventral interior. — Umbonal cavity shallow in young 

 shells, deep in old; teeth moderately strong, wide, 

 forming the margin of the delthyrial cavity; in many 

 specimens the margins of the palintrope overhang the 

 delthyrial cavity; crural fossettes shallow, oblique; 

 dental plates strongly defined, with deep umbonal cavi- 

 ties in young shells, but in old ones they become obso- 

 lete by deposition of adventitious shell. Muscle marks 

 rarely clearly defined ; in young shells the muscle field 

 is surrounded by a ridge extending from the anterior 

 ends of the dental plates, and in old shells it is on a 

 platform of adventitious shell. Adductor track broad, 

 linear; diductor track elongate; adjustor marks not 

 separable or on the sides of the delthyrial cavity. Pedi- 

 cle callist visible in the apex; anterior margin and the 

 whole interior of the shell costate in youth, but the 

 costae limited to the margins in old specimens. Interior 

 of old shells marked by irregular ovarian pits or elon- 

 gate pustules. 



Dorsal interior. — Notothyrial cavity small, shallow ; 

 cardinalia small, confined to about the posterior one- 

 fifth of the length of the shell; brachiophores grooved 

 on the inside, not distinguishable from the supports 

 which in young shells curve together to unite with the 

 floor of the valve beneath the cardinal process as in 

 Plectorthis; socket very shallow, defined by a small 

 fulcral plate, umbonal cavities prominent in young 

 shells; in old individuals all of the structures of the 

 cardinalia are completely obliterated by adventitious 

 shell. Cardinal process a simple low ridge, on each 

 side of which are the diductor impressions, much re- 

 sorbed in old shells. A low ridge divides the adductor 

 scars medianly; in young shells this elevation cor- 

 responds to the median depression in the fold of the 

 exterior, but in old shells it is a low fold of the inner 

 shell layer, extending to a point a little forward of the 

 middle; adductor scars unequal in size, the posterior 

 pair the larger, divided from the anterior adductor 

 impressions by low ridges at right angles to the median 

 ridge. Ovarian impressions in the form of pits and 

 elongate pustules. 



Geologic range. — Middle Ordovician (Black 

 River) to at least Middle Silurian (Niagaran). 



American Species 

 Unicostate group 



P. uniplkata McEwan 1919 



uniflicata uticana Rucdemann 1925 



Bicostate group 

 Subgroup A 



P. frecedens McEwan 1919 

 regularis Shalcr 1865 

 daytonensis Foerstc 1885 

 daytonensis laurelensis McEwan 1919 



Subgroup B 



P. trentonensis McEwan 1919 



trentonensis f erf Una McEwan 1919 

 trentonensis ckamflainensis McEwan 1919 



Subgroup C 



Dellhyris brachynota Hall 1843 



Orthis iiforata lynx forma reversata Foerstc 1885 



Platystrofhia camerata Twcnhofel 1928 



Subgroup D 



P. hermitagensis McEwan 1919 



Tricostate group 

 Low-fold subgroup 



P. minuta Raymond 1921 

 extensa McEwan 1919 

 globata Twenhofel 1928 

 elegantula McEwan 1919 

 elegantula triflicata McEwan 1919 

 elegantula amplisulcata McEwan 1919 

 amcena McEwan 1919 

 anicena longicardinalis McEwan 1919 

 globosa McEwan 1919 

 rhynchonellijormis McEwan 1919 

 colbiensis Foerste 1910 

 colbiensis mutata Foerste 1910 

 fracursor Foerste 1910 

 fracursnr latijormis McEwan 1919 

 ■precursor angustata McEwan 1919 



Maysville series 



P. juvensis McEwan 1919 

 fauciflicata Cumings 1903 

 strigosa McEwan 1919 

 nitida McEwan 1919 

 corryvillensis McEwan 1919 

 subiaticosta McEwan 1919 

 OrtAis morrotvensis {]amcs) 1874 



Richmond series 



P. acuminata {]3mc%) 1878 

 foerstei McEwin 1919 

 foerstei amfla McEwan 1919 

 attenuata McEwan 1919 

 clarksvillensis Foerste 1910 

 cumingsi McEwan 1919 

 annieana Foerste 1910 

 lynx moritura Cumings 1908 

 acutilirala (Connd) 1842 

 acutilirata frolongata Foerste 1910 

 senex Cumings 1908 

 elkhornensis McEwan 1919 



