GENER/\ OF THE SUBORDER PENTAMEROIDEA 



187 



European origin, and the ancestral stock is to be looked 

 for in the later Ordovician of Europe. 



Interareas are characteristic of the Orthacea and 

 Strophomcnacea of the Protremata, and in the Tclo- 

 tremata are of common occurrence only among the 

 Spirifcracea. They manifest themselves in some of 

 the Pcntameracea only and their appearance here, say 

 Hall and Clarke,"^ "may be regarded as the resumption 

 of a primitive or original character which was normal" 

 for the Protremata. 



Regarding interareas among the rostrate pentam- 

 erids, Hall and Clarke say:"^ 



Every now .ind then specimens will show a clearly devel- 

 oped cardinal area; always in Stricklandinia, frequently and 

 normally in Gyfidula, rarely and of exceptional occurrence 

 in Pentimterdla. Stricklandinia possesses so straight and long 

 a hinge, so sharply defined an area and so short a spon- 

 dylium, that it is more natural to regard this genus as the 

 accompaniment, rather than the close organic kin of the 

 other pentameroids, deriving its differentials directly from 

 those long-hinged and straight-hinged shells of the early 

 Silurian [=: Ordovician], which constitute the genus 

 Syntrofhia. 



Genus STRICKLANDIA Billings 1859 

 PI. 28, figs. 25, 27, 28; pi. 29, fig. 6 



Billings, Canadian Nat. and Geol., vol. 4, 1859, p. 132, 



figs. 8-9 {lens). 

 HalJ and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 249, 



pi. 73, fig. II. 

 Syn. Stricklandinui Billings 1863.=^ 



Genolectotype (Hall and Clarke). — 5. gasfeen- 

 w Billings 1859. 



Description. Exterior. — Shells variable in size but 

 tending to be large, elongate-oval, transversely oval, or 

 subcircular in outline ; hinge-line straight, cardinal ex- 

 tremities rounded. Lateral profile biconvex, the dorsal 

 valve usually having the greater convexity. Anterior 

 commissure rectimarginate or gently uniplicate; ven- 

 tral sulcus shallow or obsolete or raised into a low 

 fold. Dorsal fold usually present, low. Ventral inter- 

 area wide, curved, apsacline; delthyrium open. Dor- 

 sal interarea reduced, apsacline. Surface smooth or 

 multicostate. Shell substance fibrous, impunctate. 



Ventral interior. — ^Teeth small, dental plates short 

 and stout, uniting to form a small spondylium duplex. 

 Supporting septa short. 



Dorsal interior. — Internally there are two slightly 

 divergent plates which slope toward each other but do 



■* Pal. N. Y., vol. 8, pt. 2, 1894, p. 3 36. 

 " Op. cit., p. 342. 



^^ Stricklandia, a plant, does not invalidate Stricklandia 

 Billings 1859, as that author supposed. 



not unite. The inner margins of the plates are thick- 

 ened and give off long processes which form the arm 

 attachments. Adductor impressions are elongate pits 

 on the floor of the valve, the anterior pair slightly 

 divergent distally, the posterior pair narrower and 

 subparallel. 



Geologic range. — Lower and Middle Silurian. 



American Species 



Stricklandia aniicostiensis Billings 1863 



S. billingsiana {Dzwion) 1880 



S.brevis Billings 1859 



S. breviuscula (Savage) 1916 



S. canadensis Billings 1859 



S.castellana (White) 1876 



S. chafmani (Hall and Clarke) 1895 



5. circularis (Savage) 1916 



S.davidsoni {'&\\\\ng%) 1868 



S. dejormis (Meek and Worthen) 1870 



S. gasfeensis Billings 1859 



S. manitouensis (M. Y. Williams) 1919 



S.melissa (Billings) 1874 



S.multilirata {'WKxtfidd) 1877 



S.norzcoodi (Yoersii) 1906 



S . fyriformis (Sivigc) 1916 



S. fyrijormis elongata (Savage) 1916 



S. fyrijormis varicosa (Savage) 1916 



S.salteri (Billings) 1868 



5. j/ria/j (Twenhofel) 1928 



S. triplesiana (Foersle) 188 5 



Clorinda becsciensis (Twenhofel) 1928 



European Species 



A try fa lens Sowerby 1839 

 Sfirifer liratus Sowerhy 1839 

 Pentamerus microcamerus McCoy 1859 



Distinguishing characters. — Stricklandia is best 

 identified externally by its lenticular outline, wide 

 hinge-line, and well developed interarea. The spon- 

 dylium and its supporting septum are very short. The 

 cardinalia consist of two concave plates giving off long 

 brachial processes from their posterior inner margins. 



Discussion. — Billings' genus differs widely from 

 all members of the Pentameracea in several features, 

 both internal and external. The most striking external 

 variation is the width of the hinge-line, which is 

 narrow in all other known members of the super- 

 family. Along with the wide hinge-line goes a short 

 but wide interarea, seen best in the genotype. All of 

 the pentamerids retain remnants of the interarea but 

 they are confined to the margins of the delthyrium and 

 in some species are nearly obsolete. 



The spondylium is a typical duplex one, and is re- 

 markable for the abbreviation of both the septum and 

 the spoon. These features in themselves are of no 



