GENERA OF THE SUBORDER ORTHOIDEA 



113 



Subfamily CLITAMBONITIN,-E Schuchcrt and 

 Cooper 1931 



(^ Orthisinin.T Waagcn 1884) 



The typical and most prolific subfamily of the 

 Clitambonitida?, having a spondylium simplex. Em- 

 braces the following genera: 



Cl'ttamhonites Pander 



Vellamo Opik 



Clinambon, n. gen. 



Eitlattd'm Schuchert and Cooper 



Hemipronites Pander 



Pahlenella Schuchert and Cooper 



Apomatella Schuchert and Cooper 



Species (the first thirteen of Pander 1830) 



Proniles aduenden?* 



P.dta 



P. convexa 



P. excelsa 



P. humilis 



P. la/a 



P. oblonga 



P. flana 



P. fracefs 



P. frarufia 



P. rotunda 



P. tetragona 



P. transversa 



Orthisina schmidtiVMcn 1877 



? Orthisina concava Pahlen 1877 



Genus CLITAMBONITES Pander 1830 



PI. 7, figs. 17, 19-23,26 



Pander, Beitr. Geogn. Russ. Reiches, 1830, p. 7U, pi. 17, 



fig. 6. 

 Syn. Proniles Pander, Ibid., p. 71, pi. 17, fig. 6. 

 Syn. Orthisina D'Orbigny, Compt. Rend., Acad. Sci. Paris, 



vol. 25, 1847, p. 267. 



Genolectotype (Dall, and Hall and Clarke). — 

 Pronites adscendem Pander 1830. 



Description. Exterior. — Shell semielliptical, 

 hinge-line less than the greatest width of the valves; 

 lateral profile biconvex or convexo-concave; dorsal 

 valve convex; ventral valve usually subpyramidal; 

 anterior commissure rectimarginate, dorsal valve sul- 

 cate in youth only. Ventral interarea the longer, 

 strongly apsacline to procline; deltidium prominent, 

 rarely perforate; dorsal interarea the shorter, usually 

 anaciine; chilidium well developed. Surface multi- 

 costellate, commonly imbricate and spinose. CostelL-E 

 crossed by elevated concentric lines of growth. Test 

 fibrous, impunctate. 



Ventral interior. — Provided with a spondylium sim- 

 plex which is as wide as long or longer than wide, the 

 sides usually flexed inward and the base usually flat. 

 The median septum extends to about the middle of the 

 valve. The musculature was evidently confined to the 

 upper or inner side of the spondylium, the adductors 

 probably at the base, the diductor and adjuster scars 

 (if any) on the sides. 



Dorsal interior. — Notothyrial cavity small, cardinal 

 process linear, brachiophores orthoid, supported by 

 lateral extensions of the notothyrial platform, forming, 

 with the median septum, an inverted "anchor." Ad- 

 ductor field quadripartite; anterior adductor set the 

 larger. 



Geologic range. — Lower and Middle Ordovi- 

 cian of Europe. 



Discussion. — The features best characterizing 

 Clitambonites are (1) the subpyramidal ventral valve, 

 (2) the convex dorsal valve; (3) the inclination of 

 the ventral interarea; and (4) the external ornamen- 

 tation. The ventral exterior is usually slightly convex 

 or nearly flat, but in one exceptional species (C. con- 

 cava) it is concave. The ventral interarea is usually 

 catacline or decidedly procline, the angle from the hori- 

 zontal being not uncommonly more than 80', but 

 it may also be very strongly apsacline. The dorsal 

 valve is always more or less strongly convex, always 

 less deep than the ventral one, with the greatest con- 

 vexity usually located at the middle of the valve. 



The most striking external feature of Clitambonites, 

 however, is the sculpture, which sets it apart at once 

 from Vellamo, the shell of the former being strongly 

 lamellose, each interval of growth making a distinct 

 layer ("like steps or roof-tiles," Hall and Clarke, p. 

 236), and the costella produced forward as flanges, 

 so that there is a costellate border about the margins 

 of the shell when they are viewed from the inside, 

 somewhat as in A try pa (see pi. 7, figs. 19, 20). This 

 condition is approached in Vellamo by V. squamata, 

 but in that form there are differences in the spondylium 

 and external form that prevent its being placed in 

 Clitambonites. 



The deltidium of Clitambonites is typical of all this 

 family, namely, wide and highly convex and perfor- 

 ated by a large pedicle foramen. It is interesting to 

 note, however, that the foramen in the apex of mature 

 shells is rarely visible, being sealed up during early 

 maturity. It is not uncommon that the internal sur- 

 face of the deltidium is strengthened by a longitudinal 



'■' Verneuil (Geology of Russia) placed all of Pander's 

 species of Proniles except P. oblonga under the name Orlhis 

 adscendens. From our present point of view, it seems 

 apparent that \'crneuil never understood Pander's discrim- 

 inating work, and due to his great influence unjustly sub- 

 merged many of the species and generic concepts of the 

 earlier investigators. 



