PART IV. THE GENERA OF THE SUBORDER ORTHOIDEA 



The supcrfamily Orthacea erected by W'alcott and 

 Schuchert in 1908 has grown to large proportions in 

 the way of genera. Among these the presence of 

 an impunctate (m.ay be exopunctate) or a punctate 

 (alwa}'s endopunctate) shell is now seen to be of much 

 significance in taxonomy, and the genera of the old 

 division "Orthacea" are now known to be separable, 

 on the basis of punctation and certain internal features 

 of the shells, into three superfamilies, namely, ( 1 ) 

 Orthacea as here restricted, (2) Clitambonacea (for- 

 merly included in the Pentameracea), and (3) Dal- 

 manellacea. Out of the Orthacea came the (4) 

 Syntrophiacea, which may have given rise to the (5) 

 Pentameracea; both are now included in the new 

 suborder Pentameroidea. All of these superfamilies are 

 impunctate with the exception of the Dalmanellacea, 

 which are internally punctate (endopunctate). 



As the superfamilies Orthacea, Clitambonacea, and 

 Dalmanellacea are closely linked genetically and have 

 an external expression in common, i. e., transverse 

 shells with wide hinge-lines, more or less prominent 

 interareas, and usually open delthyrium, it is desirable 

 to combine them into a new suborder, Orthoidea. On 

 the other hand, the Syntrophiacea and the Pentamer- 

 acea tend more and more with time to lose their orthid 

 ex-pression and to become elongate and rostrate shells 

 with decidedly different cardinalia, and accordingly are 

 combined into the new suborder Pentameroidea. 



Therefore when we write "orthids," we refer to 

 Orthacea, dalmanellids = Dalmanellacea, clitamboni- 

 tids = Clitambonacea, syntrophiids = Syntrophiacea, 

 and pentamerids = Pentameracea. "Orthoids," on 

 the other hand, refers to the Orthoidea, and "Penta- 

 meroids" to the Pentameroidea. 



The suborder Orthoidea ranges, so far as known, 

 from the Lower Cambrian to the close of the Permian, 

 whereas the Pentameroidea begin in the Upper Cam- 

 brian and die out with the Devonian. 



Ulrich^ in discussing the musculature of the 

 Ordovician orthoids in comparison with those of the 

 Cambrian, foreshadows the classification developed in 

 this book when he says that a "natural division" of the 

 orthoids into two families is possible as follows: 



(1) Ventral muscular area small, obovate or obcordate; 

 adductors reaching front margin of area (Orthis s. s. 

 [^ Hesferorlhis], Plectorthis, Platystrophia, Hebertelta, 

 Orthostrofhia, Dalmanella [the only genus that we do not 

 include in the impunctate superfamily. This is the fore- 

 shadowing of our superfamily Orthacea, now seen to be 

 most e.isily distinguished on the basis of an impunctate 

 test]) . . . (2) Ventral muscular area large, bilobed or 

 elliptical; adductors proportionately small and more or 

 less completely inclosed anteriorly by the flabellate diduc- 

 tors {Heterorlhisy Plttsiomys? defiecta group, Dinorthis 

 [the last two genera we think are out of place here and 

 are best referred to the Orthacea], Bilobiles, Rhifidom- 

 ella, Schizofhoria, Orlholichia), [This division includes in 

 the main our Dalmanellacea.] 



Superfamily ORTHACEA Walcott and Schuchert 1908 



The evolution is thought to be as shown in Table 1 . 



Primitive Orthoidea of early Cambrian time. A 

 prolific stock with the test always impunctate (we do 

 not agree with \V^alcott's observations to the contrary). 

 Shells usually multicostellate, rarely costate and even 

 more rarely smooth or nearly so; nearly always with 

 more or less prominent interareas in both valves, and 

 wide hinge-lines; deltidia and chilidia sporadic; with 

 a simple cardinal process except in primitive genera, 

 where there is none; spondylia rarely developed, 

 pseudospondylia not uncommon. 



Range: Earliest Cambrian into Devonian. 



Includes the families: 



Nisusiids 



Protorthidje 



Billingsellidse 



Eoorthidae 



Orthidse 



Dinorthidae 



Porambonitidae 



Lycophoriidse 



Finkelnburgiids 



Plectorthids 



Skenidiida: 



' In Walcott, Camb. Brach., 1912, p. 308. 



Family NISUSIID^E Schuchert and Cooper 

 1931 



(Nisusiinx Walcott and Schuchert 1908, Wal- 

 cott 1912, Schuchert 1929) 



The most primitive of the known Protremata and 

 Orthacea, having a well developed deltidium with an 

 apical foramen in the genotype, but no dental plates. 

 Shell probably very thin. In the dorsal valve there is a 

 chilidium, and the brachiophores are rudimentary and 

 similar to those of Eoorthh or Bilimgsella. No cardi- 

 nal process is present in Nisusin, and a rudimentary one 

 only in Jamesella. 



The family embraces the genus Nisusia, and for the 

 present Jamesella is also placed here, though its origin 

 remains unknown. 



