178 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



hinge-line gently arcuate; cardinal extremities round- 

 ed; lateral profile subequally biconvex. Anterior com- 

 missure usually rectimarginate. Fold present or ab- 

 sent on both valves. Interareas obsolete; ventral beak 

 curved over the dorsal; umbo prominent. Surface 



P. cylindrktis Hall and Whitfield 1872 



P. diver gens Foerste 1909 



P. oblongus Sowerby 1812 



P.ovalis HaU 1852 



P. farvulus Savage 1913 



/>. ^ffwrw Whitfield 1882 



Table 20 



Lissoccelina 



Pentameroides 



CapeUiniella 



Pentamerus 



Holorhynchus 



Platymerella «■ 



Rhipidium 

 (costate) 



Brooksina 



Cymbidium 



/ 



Conchidium 



Harpidium 

 (smooth) 



.Virgiana 



Clorinda (GvPiDULiNiE) 



smooth or with faint undulations at the front. Shell 

 substance fibrous, impunctate. 



Ventral interior. — The dental plates are convergent 

 and with their septal plates unite to form a spondylium 

 duplex. 



Dorsal interior. — A tripartite division of the crural 

 plates is recognizable. Thin inner plates unite with 

 the walls of the valve to form lateral chambers. The 

 sockets are notches in the partitions of these chambers 

 next to the wall of the valve. The brachial processes 

 are long. The septal plates are discrete, divergent or 

 parallel, continued to the internal surface of the valve. 

 Between the septal plates a low median septum divides 

 the septal chamber into two parts. The cardinal 

 process is a callosity under the beak. 



Geologic range. — Throughout the Silurian of 

 Europe and North America and probably widely 

 elsewhere. 



American Species 



Pentamerus bisinuatus McChesney 1861 

 P. comfressui Kindle and Breger 190+ 

 P. corrugatus Waller and Davidson 1 896 



European Species 



Pentamerus borealis Eichwald 1842 



P. esthonus Eichwald 1860? 



P. gothlandicus Lebedeif 1 892 



P. Iwvis Sowerby 1812 



P. oblongus Sowerby 1812 



P. samojedicus Keyserling 1 846 



P. schmidti Lebedeff 1 892 



Distinguishing characters. — The genus Pen- 

 tamerus is characterized by its comparatively smooth 

 exterior, long spondylium duplex, discrete septal plates, 

 and tripartite cardinalia. 



Discussion. — Hall and Clarke state that Pentam- 

 erus "is an exceedingly plastic type," and limit the 

 name to smooth shells typified by P. Icevis Sowerby. 

 The latter is considered by European paleontologists 

 as the young of P. oblongus. 



The specific name P. oblongus has in America been 

 applied to a wide variety of forms, among them P. 

 oblongus subrectus Hall and Clarke. In this form 

 the septal plates of the cardinalia are united to form 

 a duplex septum which supports a shallow cruralium. 



