386 Dr. W. B. Carpenter on the Polytremata. 



rectification or further discovery, what is written under such 

 circumstances should always be considered provisional, and 

 accepted with thankfulness, inasmuch as, according to the old 

 proverb, we should not ' blame the bridge that carries us over.' " 

 To the foregoing general survey of the relations of Pol^- 

 trema and Carpenteriay I would now append two notes on 

 points of detail. 



1. I stated in my ' Introduction ' (p. 236) that while " the 

 whole shelly texture of Polytrema has ordinarily a less solid 

 character than that of Tinoporus, although formed on a plan 

 essentially the same," "we occasionally find an aggregation of 

 calcareous substance in solid pillars exactly resembling those 

 which we have seen in T. bacidatus and in Patellina Cooki.''^ 

 This last statement, although borne out by a figure, is desig- 

 nated by Mr. Carter (p. 191) as "imaginary;" and taken in 

 connexion with what follows, it certainly appears to me (and 

 I think it Avould so appear to readers in general) as if Mr. 

 Carter intended to impute to me that I had mistaken the small 

 hollow pillars that pass between the earlier-formed stories of 

 the fabric (which hollow pillars he likens, I think correctly, to 

 those of Parkevia), for solid pillars resembling those of Orbi- 

 ioides. Having forwarded to Mr. Carter the specimen on 

 which my description and figure were based, I am authorized 

 by him to state that he never intended to affirm that Polytrema 

 contains no pillars that resemble, so far as they extend, those 

 of his Conulites ( = Patellina) or of Orhitoides, but merely 

 meant that the solid pillars of Polytrema, being confined (where 

 they exist) to the superficial layers, do not correspond with 

 those of Conulites and Orhitoides, which range through their 

 entire substance. Now I had never " imagined," much less 

 affirmed, that the solid pillars of Polytrema extend through the 

 fabric ; on the contrary, I spoke of their presence as " occa- 

 sional ;" and it was in regard to their texture alone that I 

 intended to liken them to those of the other types referred to 

 — a likeness which Mr. Carter fully admits. I am happy to 

 find, therefore, that our supposed difference on this point is 

 only " imaginary.''^ 



2. On the subject of Par^ma, which is incidentally alluded 

 to by Mr. Carter, it may be well for me to state that my 

 description of it* is mainly founded on the entirely uninfil- 

 trated specimen, preserving most unmistakably its original 

 arenaceous structure, which was kindly placed in my hands by 

 Prof. Morris, and that the accuracy of this description has 

 been entirely confirmed by the examination of the gigantic 



* 'Philosophical Transactions,' 1869. 



