112 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Affinities. — Kozlowski^* has recently shown that 

 the spondylium of the pentamerids, such as Pentamerus 

 and Conchidium, has had a different origin from that 

 of Clitambonites and Skenidium. The polyphyletic 

 origin of this peculiar muscle platform is clear evidence 

 of the separate origin of the two groups. Kozlowski 

 has pointed out that the prominent development of the 

 deltidium and chilidium in the Chtambonitidffi sug- 

 gests a close relationship with the Strophomenacea, but 

 the presence of an orthoid cardinal process and other 

 features of the Orthidas shows stronger affinities with 

 the Orthacea. Before Kozlowski's paper appeared in 

 this country, the writers had also come to the conclu- 

 sion that the Clitambonitidas were primitive, aberrant, 

 and specialized orthids. We saw this relationship in 

 the ventral pallial and ovarian markings, the simple 

 septum-like cardinal process, the Orthis-Wke brachio- 

 phores and adductor field. We now know that the 

 presence or absence of a deltidium or chilidium is in 

 itself not of such great significance as was formerly 

 believed, since these features appear spasmodically in 

 many of the later genera and families. However, an 

 important feature of the clitambonitid deltidium is its 

 prominent apical foramen. Kozlowski has looked with 

 probable correctness to the Billingsellidae as the ances- 

 tral stock of the Clitambonitidas, and in this we agree, 

 since the Billingsellida: have all the necessary structures 

 by which such a transition could be accomplished. 



The writers also independently of Kozlowski saw 

 the need of separating Skenidium and its allies from 

 the Clitambonitidae. Skenidium has no deltidium or 

 chilidium, and in this respect is very specialized. The 

 internal differences, and especially the nature of the 

 cardinalia, link Skenidium with the Plectorthids rather 

 than with the Clitambonitidae. Skenidioides, probable 

 progenitor of Skenidium, undoubtedly originated in 

 Finkelnburgioy since the cardinalia of the latter are like 

 those of the Plectorthidae. 



Opik*^ also points out that the Clitambonitidae in 

 structure are foreign to the Pentameracea and places 

 them close to his new family Plectambonitidae. The 

 latter family has, however, a closer relationship to the 

 Orthacea than to the Strophomenacea, as is shown in 

 the nature of the cardinal process and other internal 

 structures, and it is not unlikely that more study will 

 establish it as an aberrant and terminal division 

 of the Orthacea. We consider the Clitambonitidae 

 as special developments, probably from the same ances- 

 tral stock, namely, the billingsellids. 



The large family Clitambonitidae is divisible into 

 the following subfamilies: 



Plectellinae Opik 



Clitambonitina Schuchert and Cooper 



Gonambonitinse Schuchert and Cooper 



"Op. cit., pp. 122-125. 



*' Acta et Comment. Univ. Tartuensis, A, vo]. 17, pt. 1, 

 1930, p. 60. 



Subfamily PLECTELLIN^ Opik 1930 



The Plectellinae are primitive Clitambonitidae with- 

 out spondylia, which apparently gave rise to the special- 

 izing Clitambonitidae of Europe. 



In his splendid monograph on the Estonian Pro- 

 tremata, Opik*^ has proposed a family Plectambonitidae 

 which he defines as follows (in translation) : 



This family embraces all Strophomenacea with a simple 

 cardinal process and strophomenoid habit, thus with con- 

 cavo-convex or convexo-concave shells. All others, with a 

 double cardinal process, form the family Strophomenidse. 



Heretofore the members of our new family have been 

 combined with the Rafinesquinina: and were placed near 

 the beginning of this subfamily. 



Opik divides his new family into three subfamilies, 

 the Plectellinje, Plectambonitinae, and Sowerbyellinae. 

 We shall concern ourselves here chiefly with the Plect- 

 ellinae, which we believe are very close to the Del- 

 tatretidae in structure. The Plectambonitinae and 

 Sowerbyellinae in their simple cardinal process show re- 

 lationships with orthoid brachiopods rather than with 

 the Strophomenacea where they have persistently been 

 classified. Here again external form appears to have 

 been the chief guide in classification. 



Within the Plectellinae Opik places Plectella Laman- 

 sky, Ingria Opik, and PaltFostrofhomena Holtedahl. 

 He believes in the uniqueness of Plectella and in this 

 he has the support of Kozlowski, who maintains this 

 to be a good genus. Schuchert (1929)*' unites 

 Plectella with Plectamhonites Pander, but in this the 

 two European authors do not concur. In our study of 

 these genera we are wholly dependent upon the litera- 

 ture and hence our conclusions may be in error. We 

 regard Palceostrofhomena as a possible member of the 

 Dinorthidae, close to Valcourea. The two remaining 

 genera are characterized by having a chilidium and an 

 imperforate deltidium. Fine radial lines mark the out- 

 side of the shells and there is no spondylium. In the 

 dorsal valve of both Plectella and Ingria the cardinal 

 process is orthoid, not plectambonitoid. In other 

 words, it does not form a tentlike structure as in 

 Sozverbyella. Furthermore, the brachiophores are 

 orthoid as in Deltatreta and are supported by lateral 

 extensions of the notothyrial platform as in Vellamo 

 and other Clitambonitina. It is our contention, then, 

 that these shells are aberrant Clitambonitidae, without 

 spondylia. Development of the Plectambonitinae and 

 Sowerbyellinae may have come from this line. In 

 America, Plectella and Ingria find their nearest rela- 

 tives in Deltatreta and Pomatotrema. These two 

 genera differ from the Plectellins in the presence of 

 an apical foramen and incomplete chilidium only, all 

 other structures being in agreement. 



*" Acta et Comment. Univ. Tartuensis, A, vol. 17, pt. 1, 

 1930, pp. 55-58. 



" Foss. Cat., Pars 42, p. 98. 



