DAVIDSON — SCOTTISH CARBONIFEROUS BRACHIOPODA. 



105 



Tamily Productid^. 



In the second volume of tlie " Geologist" tlie characters of the Troductidw 

 have been fully described and illustrated, we will, therefore, generally observe 

 that this family has been divided into four genera, or sub-genera, Produdus, 

 Aidosteges, Strophalosia, and Chonetes, but that, as they all bear too natural and 

 intimate a relation towards each other", and that the characters brought forward 

 for separating or distinguishing the four groups above named are of somewhat 

 questionable value or importance, I am disposed to consider that Aiilosteges, 

 Strophalosia and Chonetes caimot be regarded as more than snb-genera or modi- 

 fications of Productus. No calcified processes exist for the support of the oral 

 arms. In the Carboniferous rocks of Scotland Productus and Chonetes alone 

 have been hitherto discovered. 



Genus Productus. Sowerby. 1814. 



Fifteen or sixteen species of Producttis have been found in Scottish Car- 

 boniferous strata, and which have been chiefly distinguished by their external 

 shape and sculpture, for of the larger number no interiors have been hitherto 

 procured. The interior details appear to vary but little, but there exists some 

 small marked dissimilarities in certain species, and in order to avoid unnecessary 

 repetitions, we wiU at once notice their general features. 



In Productus the internal surface of the ventral valve is concave, a narrow 

 mesial ridge, originating, under the extremity of the beak, separates two 

 elongated ramified or dendritic impressions which are attributed to the adductor 

 or occlusor muscle ; and almost on a level, immediately under or above, but 

 outside of these, there exists two deep longitudinally striated subquadrate im- 

 pressions, which are in all probability due to the cardinal or divaricator muscles. 

 Impressions referable to adjustor and pedicle muscles have not been hitherto 

 found, although Mr. Hancock is of opinion that adjustor muscles must have 

 existed and been arranged, so as to keep the valves adjusted to each other, and 

 thus to have acted as a substitute for the regularly articulated hinge, which is 

 certainly absent, if not in all {^. ) at least in the generality of known species. 

 Tlie only points remaining to be noticed in connection with this valve are the 

 deep concave sub-spiral depressions visible in the interior of some thick sheUed 

 Producti, such as P. giganteus, and hoUows which were probably occupied by 

 the spiral arms. 



The internal surface of the dorsal valve is more or less convex, sometimes 

 almost flat, and presents in the middle of the hinge-line a prominent bilobed, or 

 trilobed projection or cardinal process, whose upper surface having afforded at- 

 tachment to the cardmal, or divaricator muscle. Under this a narrow longi- 

 tudinal ridge, or septum, generally extends to about half (or more) of the 

 length of the valve, and on either side are seen the ramified, or dendritic im- 

 pressions which we consider to be attributable to the adductor, or posterior 

 and anterior occlusor muscles ; these are at times situated so close together on 

 either side of the median ridge, as to render the quadruple attachment not so 

 distinct as could be desired, but they are well defined in some valves of Pro- 

 ductus longispinus, as may be seen by a reference to fig. 16 of our plate II. Out- 

 side, and in front of the muscular scars above described are the two " rehiform 

 impressions." Their surface is generally smooth; they are bounded with 

 ridges, and after dividing the occlusor muscles, proceeded in an oblique and 

 almost horizontal direction, then turning abruptly backwards, terminate at a 

 short distance from their origin. There exists also in many species, but not in 

 aU, two prominences, one on each side of the median ridge, and close to the 

 base of the muscular scars. The internal surface of Productus is covered with 

 innumerable granulations and spinose asperities. The sheU itself being likewise 

 VOL. III. 0 



