152 
The Ainerican Geologii^t. 
March, 1893 
Oamaivlla (part), Billings 1S5!I. 
Anastrophia, Hall 18t>7. 
Brachymerus, Shaler 1H(>") 
(not Dej. 1834). 
Couchidium, Linnt'- 17(i(i. 
Pontamerns, Sowerby 1S1:{. 
PentastJ^re, Blainville 1S24. 
Oypidia. Dalnian 182S. 
Pcntamerella. Hall ISCT. 
Gypidnla. Hall ISliT. 
Aniphigenia. Hall 1867. 
Camarophoria. King 1850. 
Stenochisma, Dall 1877 ; CEh- 
lert 1887 (not Conrad 1839). 
Strioklandinia, Billings 1863. 
Stricklandia, Billings 1859. 
Sieberella. GEhlert 1887. 
Antirhynchonella, Quenstedt 
1871. 
Lycophoria. Lahn.sen 1885. 
:>. Family P(ika.mi{(»M'iii».i-:. Davidson 1853. 
Porambouites, Pander 183(1. 
Priambonite?, Agassiz 1847. 
Isorhynchus, King 1850. 
Suborder Thecacea.' n. suborder. 
A])haneropegniata (part ), Productacea, Coralliopsida, and Kampy- 
lopegmata (part), Waagen 1883; Eleutherobrancliiata (part), Neu- 
mayr 1883; Cryptobrachia (part), Gray 1848. 
Famil}' Billixcjsellid.^, t n. fam. 
Billingsella. Hall 1892. 
Family Stropuomenid.t:. Kins 1846. 
Subfamily Orthothetix.*, Waagen 1884. 
Strophomenin;p (part), Waagen 1884. 
? Orthidium, Hall 1802. Kayserella, Hall 1892. 
Strophomena, Blainville 1825. Derbya. Waagen 1884. 
Hemipronites, Meek 1872 (not Meekella, White and St. John' 
Pander 1830). 1S7(I. 
*T/iei:<i, a cover. Having reference to the deltidiinu of one piece covering the delthy- 
riam or trianguhir fissure in tlie apical portion of the ventral valve. Tlie Thecacea 
dififer chiefly from the Trullacea, from which they were derived, in being without the- 
complete internal spoon or spondylium. See note" to Thullacea. 
tThose primordial species essentially orthoid in structure, but with a large deltidiuni 
and a more or less complete chilidium, have been referred to Billingsella by Hall. The- 
writer is of the impression that Billingsella or some closely allied genus gave origin to 
the OuTuiD.E, and that the former were derived from some "species also having, in addi- 
tion to the above mentioned characters, a si)ondylium, or essentially a Clitambonitfs. 
The progression towards Oiiliis from Vlitaiitbonites appears to have l)een in first elim- 
inating the spondylium Ijy attaching it to the liottom of the ventral valve, thus forming 
the dental plates and the somewhat elevated muscular area of Billingsella. The next 
step is to remove the deltidium and chilidium and to develop a more pronounced cardinal 
process to produce an 0/-//(iv. This is the course of development in geologic seciuence. 
In O/V/i/.-t (/(^/^fiiV^ Conrad, sp., there is a deltidium, which in some individuals is large 
and in otliers covers but one-half the delthyrium. In mature 0. tricenaria (Jonrad and: 
O. pectinellii Emmons, of the Trenton formation, there is present a small convex del- 
tidium and chilidium wliich in species of later faunas become nepionic characters and. 
are obsolete during nealogic and ephebolic growth. 
The iSTKOPHOMENiD.t: may also have had their origin in some form related to BHlings-- 
flla. but the data are a* yet insufticient to e<tabli<h clearly its liiie of development. 
