134 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
OrTHIDAE.—The members of this family are fairly abundant but show few 
remarkable features. It is interesting to note that many of the species have strong 
resemblances to European forms. One of the interesting European types is 
Productorthis which is represented by two species, both of which, as true Ameri- 
can representatives, far outstrip in size any of the known European species. The 
American species are not exactly like the European ones, especially in the cardi- 
nal process, but the exterior is similar. 
HESPERORTHIDAE.—This family is represented by numerous species of the 
genus Hesperorthis and, in places, hosts of Glyptorthis. 
DINoRTHIDAE.—This family had its probable origin and culmination in the 
time spanned by this monograph. The earliest genera are Valcourea and Multi- 
costella. The latter, which lacks a pseudodeltidium, is probably the more advanced 
form. The last of the American multicostellate Dinorthidae except Plaesiomys 
appear in the Lebanon and Carters formations. There they are strongly con- 
vexi-concave and without a pseudodeltidium. The origin of the family is uncer- 
tain, but it may have been derived from Hesperonomia as suggested by Ulrich 
and Cooper (1938, p. 20). 
PLECTORTHIDAE.—The earliest member of this family appears as Desmorthis 
in the Joins and Pogonip formations of the Whiterock stage. The genus is short 
ranged but may have developed into Plectorthis in the time represented by Tulip 
Creek sedimentation. Plectorthis has a long range into the Upper Ordovician. 
In general the older species of Plectorthis are more finely ribbed than those of 
the higher formations and the Upper Ordovician. Mimella is an abundant Mar- 
mor form which extends into the Dryden and Wardell but then gives way to Dole- 
roides which is the probable forerunner of Heberiella. No origination stock for 
the Plectorthids can now be postulated other than the Finkelnburgiidae as indi- 
cated by Ulrich and Cooper (1938, p. 18). 
SKENIDIIDAE.—This group appears in the formations deposited in Marmor 
time, and some of its members are fairly long ranged. Skenidioides appears in 
Porterfield time and extends into the Silurian. In this family, too, are placed a 
smooth to plicate form of skenidioid which has modified cardinalia, Tropidothyris. 
Scaphorthis and Phragmorthis are also placed within the family. These are char- 
acterized by cruralia and the latter by an enormous median septum. Scaphorthis 
has the profile and ornamentation suggesting a dalmanelloid, but the shell is 
definitely impunctate. Origin of these shells is obscure indeed, and they may 
not be a natural group as now placed. 
TRITOECHIIDAE.—This family is represented by the unusual genus Eremo- 
toechia that is essentially a strongly biconvex Tritoechia having the external 
appearance of Mimella. In spite of appearances the genus has the apically per- 
forated pseudodeltidium (the foramen is smaller than in Tritoechia) and cardi- 
nalia of the Ordovician (Canadian) genus. The few specimens in the collection 
do not show development of a pseudospondylium such as that often exhibited by 
Tritoechia. No other shells related to this one were seen, and this may be the 
end of the stock. 
CLITAMBONITIDAE.—This family is represented by Atelelasma which is spe- 
