﻿ORTIIIS. 



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touch each other, but are in general moderately separate. Exteriorly, both valves are 

 closely covered with numerous fine, thread-like, rounded, radiating stri;e, which increase 

 in number by interstriation and bifurcation, at variable distances from the beaks, and at 

 intervals the stria? themselves slightly augment in thickness and prominence, producing 

 small, hollow, thread-like, tubular spines, which become more numerous towards the 

 margin. The intimate shell-structure is perforated by innumerable canals, of which the 

 exterior orifices, in the shape of minute punctures, cover the entire surface of the valves. 

 In the interior of the ventral valve the teeth arc prominent, and the dental plates extend 

 to some distance along the bottom of the shell ; the muscular impressions occupy an 

 elongated oval or saucer-shaped cavity with raised margin; this cavity is divided along the 

 middle by a small, rounded, or angular ridge. The adductor or occlusor muscle is central, 

 and leaves a small but not always clearly defined depression on either side of the mesial 

 ridge, while it is probable that each of the larger cavities (on either side of the mesial 

 ridge and adductor impression) may have been formed by, or afforded attachment to, 

 the cardinal or divaricator and the ventral adjustor muscles of Hancock ; the last-named 

 muscular impression being indistinct, but situated close to the outer margin of the saucer- 

 shaped cavity above described. The small triangular scar at the bottom of the fissure I 

 am inclined to attribute to the pedicle-muscles. In the dorsal valve the fissure is partly 

 occupied or divided by a moderately produced shelly prominence or cardinal process, to 

 which were, in all probability, attached the divaricator muscular fibres. The inner socket- 

 walls are prolonged to some distance into the cavity of the shell, and form two curved 

 blade-like brachial processes, to which the spiral arms may in all probability have been 

 attached, and to the inner surface of these blade-like processes the dorsal adductor was in 

 all probability fixed, while under the cardinal process above described a rounded central 

 ridge separates the quadruple adductor or occlusor impressions. The vascular impressions 

 are more or less observable, and consist of six principal trunks in the dorsal valve, two in 

 the ventral, the external branches turned outwards and backwards, inclosing the ovarian 

 spaces. Proportions variable ; two specimens measured — 

 Length 14, width 17, depth 9| lines. 

 „ 15 „ 16 „ 11 „ 

 Obs. The same uncertainty already expressed with reference to the specific value of 

 Strept. nmbraculum and St. crenistria may be here repeated in connection with Orthis 

 resupinata and O. striatula. The greater number of palaeontologists (as may be seen 

 from our list of synonyms and references, and which might have been made still more 

 numerous) would maintain the Devonian 0. striatula as a distinct species, while Bronn, 

 Phillips, and Morris, would place the term 0. striatula among the synonyms of 0.. resu- 

 pinata. Any how, if 0. striatula is to be considered distinct from 0. resupinata it cannot, 

 as some have proposed, retain the specific denomination of 0. connivens (Phillips), since 

 this last designation was made use of in 1836, while the name striatula dates as far back 

 as 1813. 



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