BILLINGSELLID^. 725 



Order PEOTEEMATA Beecher. 



[Tipi, early; and rp^/za, perforation.] 



Superfamily ORTHACEA Walcott and Schuchert. 



Family BILLINGSELLID^ Schucliert. 



SutofamUy NISUSIIN^ Walcott and Scliucliert. 



Genus NISUSIA Walcott.a 



Billingsella Hall and Clarke (in part), 1892, Nat. Hist. New York, Paleontology, vol. 8, pp. 230-231. (Described 



and discussed as a new genus. As described the genus includes species that are now referred to Nisiisia.) 

 Nisusia Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 247-248. (Described essentially as below as a new genus.) 

 Nisusia Walcott, Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 210. (Characterized.) 

 Nisusia Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 147. (Classification of genus.) 



Form subquadrate to transversely semioval. Shell structure dense with a minutely 

 granular groundmass with minute pores penetrating some of the lamellae forming the shell. 

 The pores vary greatly in number and arrangement. Surface with narrow, radiating ribs 

 that support irregularly distributed spines on their crests. 



Ventral valve elevated and usually much more convex than the dorsal, and in most species 

 a mesial sinus appears on it. Area high, vertical or inclined backward; delthyrium large and 

 partly closed with a convex deltidium; teeth strong and supported by dental plates that 

 extend outward, also inward, forming on the inside a shallow spondyhum. Dorsal valve 

 moderately convex with only a trace of a median elevation; usually the surface is convex, but 

 it may be slightly concave from the umbo to the margins; crura well developed, with the crural 

 plates extended so as to form a shallow cruralium. No traces of a cardinal process have been 

 observed in the dorsal valve. 



Type. — Orthisina festiTiata Billings. 



Oiservations. — A comparison of the area of the ventral valve of Nisusia with that of the 

 same valve in Billingsella shows in each a convex deltidium partly covering a large trian- 

 gular delthyrium. In Billingsella the upper extremity of the deltidium is perforate and it 

 is probably so in Nisusia, but on this point there is an uncertainty, as no specimen has been 

 found clearly showing the pedicle opening. It may be that the deltidium of Nisusia was per- 

 forate in the younger stages of growth and subsequently filled by shell growth. The strong 

 vascular sinuses of the ventral valve of Billingsella are absent in Nisusia, as well as the tripartite 

 division of the umbonal cavity. The interiors of the dorsal valves of the two genera show 

 marked differences. The cardinal process is well developed in Billingsella but not in Nisusia. 

 There is a small, shallow cruralium in Nisusia but none in Billingsella. The dorsal valve of 

 Nisusia has a clearly developed cruralium, but not any trace of a cardinal process. The ventral 

 valve of Billingsella shows a tendency to form a spondylium resting on the interior of the 

 valve, a character unltnown in Nisusia. 



The general form of Nisusia is not unlike that of Billingsella except that its ventral valve 

 has a more erect beak and cardinal area. These comparisons include such species of Billingsella 

 as B. color ado ensis, B. plicatella, and B. exporrecta, but not B. dice (PI. CI, figs. 8, 8a-c). Of 

 Nisusia, they include N. festinata and some of the subgenus Jamesella, such as N. {J.) per pasta, 

 erecta, and amii. 



Billingsella and Nisusia both occur in Lower Cambrian strata, but Billingsella has its 

 greatest development in the Middle Cambrian and Nisusia in the Lower Cambrian. It is not 



a The synonymy for this genus does not give a complete record of the various genera under which the species now included in Nisusia were 

 formerly placed; it gives only those references in which the genus is discussed or described. To complete the record the following mere generic 

 references are listed: 



Orthisina de Verneuil and Barrande [1860, p. 533]. 

 Orthisiita Billings [1861b, p. 10; lS61c, p. 949; 1862b, 



p. lO.-i; 1862e, p. 221; 1863, p. 284]. 

 Orthisina Mallada [1875, p. 32]. 



OrJfc'sma Walcott [188Bb, pp. 120 and 121; 18890, 



p. 442; 1891a, p. 613]. 

 Orthisiita Tate [1892, p. 1S5]. 

 Billingsella Schuchert (1897, p. 158]. 

 Orthisina Matthew [1902c, p. 109]. 



