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STROPIIOMENA. 121 



denotes that a pedicle muscle must have existed, although the foramen became closed as 

 soon as the animal found it could dispense with the moorings required during the early 

 stages of its development. 1 



In the interior of the dorsal valve the cardinal process is divided into two lobes, and 

 not connate with the diverging socket-ridges. From the base of this a slight median ridge 

 runs down and separates the two pairs of adductor or occlusor scars, which are bordered 

 by prominent ridges. The vascular impressions consist of large primary vessels which 

 run at once direct from near the centre of the valve, to a short distance of the frontal 

 margin, when they become reflected on either side to surround the ovarian spaces, and, 

 near the margin, some of the vessels bifurcate several times. 



Dimensions very variable: a large example measured, length 17, breadth 33 lines. 



Obs. As -may be observed from the synonyms and references given (and which 

 could have been considerably increased in number), this species has already been located 

 in at least six genera, and has received twelve or more specific denominations. It is not, 

 however, surprising that before the class had been sufficiently investigated, and while 

 efforts where being made to value and define the different interior characters, as a means 

 of classification, that geologists and palaeontologists should have felt uncertain where to 

 locate this and other species, so that the different generic denominations may thus be easily 

 and naturally accounted for. We next come to the specific denominations ; and here we 

 open out a field susceptible of much divergence in opinion, for the preconceived opinions of 

 some palaeontologists and geologists make them averse to recognise the existence of the same 

 species in so extensive a vertical range as the Silurian, Devonian, and Carboniferous periods ; 

 and although the greater number of palaeontologists have admitted, in their various works, 

 that it was scarcely possible to separate the specimens of the shell as found in the three 

 great divisions of the Palaeozoic period, they still preferred to retain for each a distinct specific 

 denomination : and this will explain the principal cause of the many names the shell we are 

 now describing has received at different periods and from different hands. 



In 1836, while describing the Carboniferous shell, Professor Phillips observes : — 

 "I cannot distinguish this from the Dudley species, L. depressa " and the same author 

 begins the description of his Prod, analoga by stating that, " Perhaps this is really differ- 

 ent from the last {L. depressa), but it is difficult to fix on characters." In 1843, Professor 

 de Koninck observed, that it was impossible to distinguish the Silurian Stroph. rhomboidalis 

 from the Carboniferous Lept. analoga, and he united the two under a single denomination. 

 In p. 389 of his ' British Palaeozoic Fossils,' while alluding to Lept. analoga, Professor 

 M'Coy remarks, that " the shell in the Middle and Upper Palaeozoic rocks can scarcely 

 be considered as a distinct species from L. depressa of the Lower Palaeozoic strata, and of 

 which it is only a variety, not separable when the extremes are compared, but generally 



1 According to Darwin, a muscle does not exist simply because it was created, but because a necessity 

 for it existed originally, and having once come to be of general use, traces of it might remain (as a rudi- 

 mentary organ) in after-times, when the creatures had ceased to use it. 



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