112 



THE GEOLOGIST. 



this interpretation of the origin of the reniform impressions, as he 

 does not see how they can have anything to do with the vascular 

 system, properly so called, and to which they have been attributed by 

 various authors. They could not possibly have been produced by 

 pallial or ovarian sinuses ; for, if s(5, we should have expected to find 

 them also in the ventral valves."^ 



We will now add a few words relative to the probable mode of 

 existence assumed by the Prochictida3, and which does not appear to 

 have been the same for all the species of which the family is com- 

 posed. The opinion entertained by some palaeontologists that the 

 shell was suspended by muscular fibres issuing from tubular cardinal 

 spines, or from between the margins of the shell, are highly im- 

 probable, and unsupported by any acceptable evidence. It is, however, 

 probable that some of the species were free and unattached, while 

 others show clear evidence as to their having adhered to marine 

 bodies by the beak of their ventral valve {e.g. ^trophalosia and 

 Aulosteges). D'Orbigny supposes that the animal of Froducta lived 

 on soft sea-bottoms, lying with the smaller or dorsal valve uppermost, 



* In a letter I have recently received from Mr. Hancock, there is the following 

 passage : — " The idea that the reniform impressions gave support to the arms does 

 not appear inconsistent with the opinion expressed by you and others, namely, 

 that a portion of each arm was arranged spii'ally, and occupied the hoUows in the 

 ventral valve. I am quite inclined to believe that these reniform callosities gave 

 support to the first or basal portion of the arms. The arms may aftei'wards have 

 become free, and have formed more or less incomplete spirals, and may have fitted 

 into the subspiral cavities of the ventral valve in Producta gigantea^ &c. In some 

 other species no spirals may have been developed, and the extremities of the arms 

 may have been disposed in some other manner. In Thccidium the terminal por- 

 tions are variously arranged ; and this may have been the case m the Productidce. 

 It is impossible not to be struck with the resemblance of the reniform impressions 

 in fig. a of your PI. IV., to the ridges supporting the arms of Thecidium in PI. VI. 

 fig. 42, of your General Introduction." 



Professor King explains his views regarding the origin of the reniform impres- 

 sions in tlie following words, Avhich I think it well to reproduce in this place, that 

 the reader may have before him the reasons adduced by those who would connect 

 tiie above-mentioned impressions with the vascular system, as well as of those who 

 attribute them to the ridges supporting the arms : — 



" Taking Leptcena analoga and Produdvs horridus, as examples illustrating the 

 characterism of the vascular system of their respective families, it may be predi- 

 cated of Stro2)h omen idee, that tlie primary pallial vessels arc more or less confined 

 to the medio-longitudinal region of the valves ; and of Productidce, that they 

 strike oft' at the moment tliey'issue from between the muscular scars, ui a lateral 

 direction, running for some distance nearly parallel to the cardinal line, then 

 curving forward and round towards the centre, and finally returning to nearly then- 

 origin. Looking at the vein-like fine bounding the reniform lobes of P?-0(:/wc^((s 

 horridus [see the woodcuts] and P. scminiiculafus, I cannot but think that these 

 structures are each due to a recur\ iug vessel, rather than to an expanded and 

 simply projecting vascular organ, as appears to be the case in Criopus {Crania).'" • 



