180 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



consin. It forms a remarkable example of reversion 

 of relative convexity of the valves and from its com- 

 paratively smooth shell is clearly an offshoot from 

 Pentamerus. The ventral median septum is rather 

 long, reaching about to the middle of the valve. It 

 extends practically to the beak, showing the spondylium 

 to have been supported for its whole length. The 

 spondyhum as shown on interiors is, however, rather 

 short, shorter than usual in Pentamerus. In the dorsal 

 valve the septal plates are rather closely placed and 

 nearly parallel. The portion bounded by them is 

 divided by a low median ridge as is usual in the 

 pentamerids. 



Genus HOLORHYNCHUS Kiaer 1902 

 PI. 27, fig. 20 



Kiaer, Norg. geol. unders. aarbog for 1902, 1902, p. 68, 

 figs. 1-7, pp. 103-110. 



Genoholotype. — H. giganteus Kiaer 1902. 



This name was proposed by Kiaer for shells ex- 

 ternally like Pentamerus ohlongus Sowerby except that 

 they are transverse and have a free spondylium in 

 the ventral valve. The structure of the dorsal valve 

 is like that of Pentam,erus. 



Geologic range. — The only known form, the 

 genotype, is from the Silurian of Norway. 



Genus HARPIDIUM Kirk 1925 



Kirk, Proc. U. S. Nat. Mus., vol. 66, 1925, pp. 1-5, pi. 1, 

 figs. 1-6, pi. 2, fig. 7. 



Genoholotype. — K. insignis Kirk 1925. 



Description. Exterior. — Shells large, longitudi- 

 nally subtriangular ; hinge-line narrow; cardinal ex- 

 tremities obsolete. Lateral profile unequally biconvex, 

 the ventral valve usually having the greater convexity. 

 Anterior commissure emarginate, sulci of both valves 

 rather shallow. Interareas obsolete or exceedingly 

 narrow. Beaks of both valves strongly arched, the 

 ventral one bent over that of the dorsal. Beaks acutely 

 pointed; umbo swollen; delthyrium covered by a 

 prominent deltidium. Shell thick, fibrous, impunctate. 



Ventral interior. — Spondylium duplex long and nar- 

 row, septum short. 



Dorsal interior. — As in C onchidium and Pentam,- 

 erus, there are three sets of plates. The inner ones 

 form chambers at the back of the valve ; the outer ones 

 bear long brachial processes and are supported by low 

 septa. The septal plates are subparallel and always 

 discrete. 



Species 



H arfidium insignis Kirk 1 92 5 

 H.latus Kirk 1925 

 H. rotunius Kirk 1925 



Distinguishing characters. — Harpidium may 

 be recognized easily by its smooth exterior, strongly 

 incurved beaks, unusually long spondylium and very 

 abbreviated septum. 



Discussion. — This very interesting genus has ob- 

 vious similarities to Conchidium. These are seen in 

 the elongate beaks, the disproportionate size of the 

 valves, and the whole physiognomy of the shell. Fur- 

 ther, Harfidium and Conchidium have the peculiar 

 concave type of deltidium, the only instances of its 

 occurrence in the Pentameridas. 



Despite these obvious relationships there are two 

 points of difference of considerable significance. One 

 of these is the smooth surface which, together with the 

 external configuration, makes this shell unique. The 

 other difference is found in the abbreviated septum. 

 In typical Conchidiufn the septum extends for nearly 

 the full length of the valve and supports the spondylium 

 for nearly its entire length. In Harpidium, on the 

 other hand, the septum is unusually short and the 

 spondylium hangs free for nearly its whole length. 

 In one specimen the length of the septum is 17 mm., 

 that of the spondylium about 54 mm., with a greatest 

 width of 1.5 mm., while the length of the shell meas- 

 ures 75 mm. One wonders how the spondylium, 

 hanging free as it does, can be strong enough to serve 

 as the seat of attachment of the opening and closing 

 muscles of the shell. Strength is evidently obtained 

 by the narrowness of the spondyhum and the broad 

 surface of muscle attachment. 



Of most interest in H arfidium is the deltidium, 

 which Kirk described as "convex, elevated, deltidial 

 plates." These are, however, not deltidial plates, but 

 are the protruding sides of the deltidium, which is a 

 continuous plate convex postero-dorsally. This struc- 

 ture is like that of Conchidium (see pi. 29, fig. 4; 

 t. fig. 25). 



Harpidium is evidently restricted to the Silurian of 

 the Pacific province in Alaska. At Louisville, Ken- 

 tucky, occurs a species, usually called Pentamerus per- 

 gibbosus Hall and Whitfield, which is externally like 

 Harpidium in being smooth, but is like Conchidium 

 internally, and for this shell, which represents a stage 

 of pentameroid evolution, we have proposed the new 

 name Lissocaelina. 



Costatje or Costate Pentamerina: 



Genus RHIPIDIUM Schuchert and Cooper 

 1931 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 249. 



Genoholotype. — Pentamerus knappi Hall and 

 Whitfield 1872, 24th Rept. N. Y. State Cab. Nat. 

 Hist., p. 184. 



This name is proposed for rather flattish shells that 

 have commonly but erroneously been classed with 



