184 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



sets of plates is not entirely clear. The inner plates 

 appear to be exceedingly short, making small chambers 

 at the rear as in Conchtd'tum. In front of the inner 

 plates the septal ones are low and bear elongate 

 processes as is usual in pentamerids. 



It is difficult to trace the lineage of Cymhidium. 

 From its external form it would appear to be related 

 to Conchidium {"biloculare") but the spondylium is 

 much wider than is usual in that genus. The loss of 

 the ventral septum appears to be rare among pentam- 

 erids, and in the orthids with spondylia is seen only in 

 the Middle Cambrian Protorthis and Loferia. 



Genus PLATYMERELLA Foerste 1909 



PI. 27, figs. 2,3,5, 11 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 14, 1909, 

 p. 70, pi. 1, figs. lA-D; vol. 19, 1920, p. 223, 

 pi. 23, fig. 5. 



American Species 



Platymerella man?:ie?!sis Foerste 1909 

 Conchidium crassiflica Hall and Clarke 1 895 



Distinguishing characters. — Platymerella was 

 distinguished by its nomenclator as unique for the fol- 

 lowing reasons: (1) absence of a straight hinge mar- 

 gin, (2) non-galeatiform exterior, (3) ventral and 

 dorsal beaks approximate, (4) median septum short. 



Discussion. — Some time after the appearance of 

 the original description of Platymerella, Dr. Foerste 

 redescribed the genus on the basis of new material 

 showing perfectly the character of the internal struc- 

 ture, which had hitherto been unknown. As shown 

 by these specimens, the genus has essentially the same 

 structure as Pcntamerus, but the plates in both valves 

 are unusually short, actually making it difficult to 

 distinguish the ventral from the dorsal, as stated by 



12 5 4. ^ 



F,c. Z\.—Cymbidium acutum Kirk. Sections cut through the beak of a dorsal valve, showing pentameroid character 

 of the septal plates. Distance from beak: 



1 — 1 mm. + — 5 mm. 



2—3 5—6 



3—4 



Genoholotype. — p. manniensis Foerste 1909. 



Description. Exterior. — Longitudinally suboval, 

 hinge-line narrow; cardinal extremities rounded. 

 Lateral profile subequally biconvex; anterior commis- 

 sure rectimarginate. Fold and sulcus indistinct, or 

 ventral sulcus shallow and dorsal fold low. Beaks 

 subequal, incurved. Surface multicostate ; shell sub- 

 stance fibrous, impunctate. 



Ventral interior. — Spondylium duplex short, septum 

 short. Spondylium directed antero-dorsally. Ovarian 

 areas marked by elongated pustules. 



Dorsal interior. — Cardinalia confined to the poste- 

 rior, exceedingly short. Septal plates short, discrete or 

 united by extra shell deposit to form a small chamber 

 which is U-shaped in cross-section, making a pseudo- 

 cruralium. Brachial processes as in Pentamerus. In 

 front of the pseudocruralium is a long, slender, but 

 low median ridge. The adductors are elongate impres- 

 sions on each side of the median ridge. 



Geologic range.— Early (and ? Middle) Silurian. 



Foerste. The published description of the genus leaves 

 little to be added now that the interiors are known. 

 However, Foerste figures and describes two types of 

 shells in Platymerella. In one of these, the septal 

 plates of the cardinalia are united at their junction 

 with the valve by extra testaceous substance which 

 forms a pseudocruralium, as may be seen in Foerste's 

 figures 5E and 5F. The other type appears in figures 

 5G and 5H; it shows two elongate septal plates ex- 

 tended for a considerable distance along the inner 

 surface of the valve, and strongly resembles Pentam- 

 erus. In the type which forms a pseudocruralium 

 there is no trace of the extensions of the septal plates 

 forward, and, on the other hand, in the type with the 

 elongate, parallel plates there is no evidence of a median 

 septum. There are two possible explanations: (1) 

 we are here dealing with homceomorphs, one type 

 representing Platytnerella and the other being refer- 

 able to Pentamerus; (2) we may look at the problem 

 from another angle and maintain that both types belong 

 to the genus Platymerella, the form with the pseudo- 



