PLATE 4 



Figs. Cyrtonotella semicircularis (Eichwald) 

 1,4,5,11. — Ventral, dorsal, posterior, and lateral views, 

 showing multicostate exterior, short interarea, and 

 concave dorsal valve. 

 Ordovician (Echinosphaerites Is.), Popovka, near 

 Leningrad, Russia. Cat. No. S 169. x 5. 



Glossorthis tacens Opik 



2^ 8. — External and internal views of the same dorsal valve. 

 The latter illustrates the cardinal process and the 

 thickening or swelling just in front of it that supports 

 the brachiophores. The latter structures are broken 

 off near the sockets so that their full length is not 

 revealed. Anterior adductor scars bipartite as in 

 Productorthis and many other orthoid genera. 



7, 9. — External and internal views of an exceptionally well 

 preserved ventral valve. Fig. 9 illustrates vvell the 

 pseudospondylium and also the various muscle impres- 

 sions. This pseudospondylium is entirely a secondary 

 feature, being best developed in mature shells, and is 

 formed as a deposit under the muscle attachments. In 

 G. extensa, the pseudospondylium is not developed to 

 such a marked degree. For details of the ventral 

 interarea, see fig. 28. 



12. — ^Ventral valve, tilted upward to afford a better view 

 of the pseudospondylium. The short septum visible 

 in front, and the thick rim, are adventitious deposits 

 simulating a spondylium. 



28. — Enlarged view of interarea of ventral valve of the 

 same specimen illustrated in fig. 9, x 3, showing pon- 

 derous teeth and heavy later-il (deltidial? ) plates 

 along delthyrial margin. On the right side of the 

 delthyrium, one of these is well shown, the suture 

 being emphasized by slight fracturing. 

 Mid. Ordovician (Cj), Kohtla, Estonia. Cat. No. 

 S2100. All except fig. 28 x 1.5. 



Glossorthis extensa (Verneuil non Pander) 

 3^5^ 10. — Dors.-il, posterior, and lateral views of the same 

 individual. 

 Ordovician (Glauconite Is., B,), Popovka, near Len- 

 ingrad, Russia. Cat. No. S 161. x 1.5. 



Productorthis parallela (Pander) 

 15^ 16. — Internal and external views of posterior of the 

 same individu.il as in pi. 3, figs. 1 1, 12._ Note unusual 

 cardinal process and cuplike adventitious shell sup- 

 porting brachiophore in fig. 15, and umbrella-like 

 chilidium in fig. 16. 

 Ordovician (Glauconite Is., B,), Popovka, near Len- 

 ingrad, Russia. Cat. No. S 127. x 3. 



Figs. Hesperorthis tricenaria (Conrad) 



13, 14, 25. — Ventral, dorsal, and posterior views of a com- 

 plete individual. Contrast broad interareas of figs. 14 

 and 25 with shortened ones of Orthis, figs. 7 and 8 

 of pi. 2. 

 26. — Smaller specimen preserving remnantal chilidium and 

 deltidium. 

 Ordovician (Black River), Cannon Falls, Minn. 

 Cat. No. S199. x 1.5. 

 17. — Internal view of dorsal valve, showing chilidium, 

 brachiophores, strong median elevation, and internal 

 marginal ribbing. This type of marginal ribbing, 

 with a cleft internal rib corresponding to the stria 

 (groove) of the exterior, is common to many genera 

 of the Orthida;. x 2. 

 18. — Same shell, tipped forward to give a better view of 



the convex chilidium. x 2. 

 29. — Enlargement (x 4) of cardinalia of fig. 1 8, to show 

 chilidium and brachiophores in greater detail. These 

 brachiophores are typical of the ''Orthis" type, long 

 and pointed, grooved on the inside, triangular in sec- 

 tion, the outer sloping face forming resting places for 

 the teeth of the ventral valve, the crural fossette of the 

 ventral tooth resting on the carinate, postero-ventral 

 ridge of the brachiophores. 

 Ordovician (Trenton), St. Paul, Minn. Cat. No. 

 S210. 

 19, 27. — Ventral interiors, showing long interarea and short 

 deltidium. Notice distinct suture-lines between mar- 

 gins of delthyrium and deltidium. In fig. 27 the 

 slender ridge dividing the main pallial trunks of the 

 ventral valve is visible, and a few of the elevated lines 

 of the ovarian areas can be seen near the hinge 

 margins. 

 2 1 . — Dorsal interior, showing cardinal process and brachio- 

 phores. Anterior portion of the cardinal process bear- 

 ing a shallow groove. 

 Ordovician (Trenton), ScuUsburg, Wis. Cat. No. 

 S176. Fig. 19 X 1.5; figs. 21 and 27 X 2. 

 20. — Ventral valve, showing details of interarea. This 

 specimen illustrates a feature of unusual interest: the 

 delthyrium is extremely narrow, having been dimin- 

 ished by the growth of margin.il plates inside the 

 teeth. (See fig. 28.) Deltidial plate an arch under 

 delthyrial margins. 

 Ordovician (Black River), Minneapolis, Minn. 

 Cat. No. S 2101. X 1.5. 



Hesperorthis davidsoni (Verneuil) 

 22, 23. — Dorsal and ventral exteriors of a well preserved 

 individual. In fig. 22 perforations are visible on the 

 surface of the costa:, but these extern.il apertures are 

 not to be confused with puncts (endopunctae). 

 24. — Dorsal interior, showing muscle-scars and cardinalia. 

 Silurian (Gotlandian), Gotland, Sweden. Cat. 

 Nos. S203, S204 (fig. 24). x 1.5. 



202 



