114 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



thickening or low septum. The spondylium is sessile 

 at its posterior extremity but is elevated on a septum 

 in its median portion and free at the front end. The 

 umbonal cavities commonly have some filling but never 

 to the degree shown by Vellamo diversa from Anticosti. 

 It was Hall and Clarke^* who revived Pander's 

 " Klitamhonites" as a generic term, and restricted the 

 name, as they thought, "pretty nearly" to Pander's 

 term Pronites. They, however, did not appreciate the 

 features of Gonambonites, nor of the tribe of Orthisina 

 verneuili which has been recently made the genotype of 

 the genus Vellamo by Opik. 



Genus VELLAMO Opik 1930 



PI. 7, figs. 5, 8-11, 14-16, 18, 24, 25, 27-32; 

 pi. 29, figs. 8, 14 



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

 1930, p. 212. 



Genoholotype. — Orth'ts verneuili Eichwald 1841, 

 Urwelt Russlands, II, p. 51, pi. 2, figs. 3-5. 



Description. Exterior. — Shell narrowly semi- 

 elliptical, hinge-line straight, as wide as or wider than 

 the total width of the shell; cardinal extremities usu- 

 ally acutely angular. Lateral profile piano- to slightly 

 concavo-convex; anterior commissure rectimarginate 

 to broadly sulcate. Ventral palintrope long, strongly 

 apsacline to catacline but never procline. Dorsal pal- 

 intrope shorter, anacline. Deltidium perforate in ma- 

 ture forms, with the apical foramen sealed in old in- 

 dividuals. Surface coarsely costellate, the costellse 

 being crossed by fine elevated lines of growth. Test 

 fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep, teeth 

 strong, forming a thickening along the sides of the del- 

 thyrial margin; crural fossettes shallow. Spondylium 

 slightly depressed so that it is usually wider than high; 

 median septum low but long, extending nearly to the 

 front margin. Internal lateral spaces marked by irreg- 

 ular granules. 



Dorsal interior. — Notothyrial cavity small, shallow, 

 divided into two chambers by the cardinal process. 

 Brachiophores orthoid, supported by lateral growths 

 anteriorly, formed by the swelling of notothyrial plat- 

 form. Median ridge thick and broad, extending to the 

 front of the muscle area; sockets shallow; adductor 

 field quadripartite, the anterior set being the larger 

 and divided from the posterior pair by low ridges at 

 right angles to the median ridge. An oblique pallial 

 ridge is given off from the antero-lateral extremities 

 of the anterior adductor scars. Internal surface 

 marked by low granules. 



Geologic range. — Middle and Upper Ordovician 

 (Chazy to Richmond). 



«" Pal. N. Y., vol. 8, pt. 1, 1892, p. 233. 



American Species 



Orthisina diversa Shaler 1865 



Clitatnbonites americanus Whitfield 1877 



C diversa altissima WincheU and Schuchert 1 893 



C. rogersensis Foerste 1910 



C. ruedemanni Raymond 1 92 1 



C. trentonensis Raymond 1 92 1 



aff. C. multicosta (Hudson) 



European Species 



Orthisina adscendens Davidson (Brit. Foss. Brach., vol. 3, 



pt. 7, p. 278) (non Pander). 

 O. comflectens 'Wivcid.n 1907 

 O. emarginata Pahlen 1877 

 O fyramidalisYztii&n X'iill 

 O. squamata Pahlen 1877 

 O. verneuili wesenbergensis Pahlen 1877 

 Orthis verneuili Eichwald 1841 

 Clitambonites comflectens albida Reed 1917 

 C. humilis Fuchs 

 Skenidium grayitiT>a.vi6i&on 1883 

 S. shallochiense Davidson 1883 

 Vellamo farva Opik 1930 

 V. fyramidalis arcuato Opik 1930 

 V . fyramidalis fahleni Opik 1930 

 V . fyramidalis simflex Opik 1930 

 K. wWwjOpik 1930 



Distinguishing characters. — Vellamo is char- 

 acterized externally by the wide hinge-line, apsacline 

 palintrope, concavo- to plano-convex profile, and non- 

 imbricate shell sculpture. 



Discussion. — From the above it is apparent at once 

 that Vellamo differs in important details from Clitam- 

 bonites. In the former the palintrope is usually long, 

 flat or curved noticeably, as a rule moderately or 

 strongly apsacline, and only very rarely catacline. 

 Clitambonites altissimus and C. trentonensis among 

 American species have nearly catacline pahntropes. 

 Vellamo differs further from Clitambonites in not 

 possessing the strongly lamellose or frilled ornamenta- 

 tion. The surface sculpture is like that visible in the 

 umbonal region of Clitambonites as pointed out by 

 Opik. The type of ornamentation in Clitambonites is 

 approached in Vellamo only by Orthisina squamata 

 Pahlen, which is placed here because of internal 

 similarities. 



Internally it is difficult to define any substantial 

 difference between the two genera. The spondylium 

 of Vellamo is usually wider than long and this de- 

 pressed appearance is in contrast to the elongate or 

 narrow one of Clitambonites. Inside the dorsal valve 

 the fundamental structures of the two genera are essen- 

 tially the same, the only perceptible variations being in 

 a rather thicker notothyrial platform in Vellamo and 

 the flange-like border around the internal margin of 

 Clitambonites. Opik points out further that the chi- 

 lidium of Vellamo is relatively short and wide and the 

 muscle-scars generally wider and shorter, but there 

 appears to be little generic value in these features. 



