290 GENERAL SUMMARY TO 



The Shell Structure. 



Microscopical investigations in connection with the intimate structure of the shell of 

 the Brachiopoda have led to very important results. The subject is a large one, and has 

 been admirably worked out by several able and accurate observers. The structure of the 

 shell is complicated, and different in some genera and species. It is fibrous without any 

 perforations in Rhynchonella. In others it is vertically traversed throughout by canals 

 varying in size, &c, while in some the perforations do not traverse the shell from 

 surface to surface, but are restricted to a portion of its thickness only. It is therefore 

 not sufficient to say that the shell is perforated ; one must describe in what manner and 

 to what extent the perforations extend. The numerous careful observations already 

 effected during the last forty years have thrown very considerable light upon the subject, 

 and there remains a good deal more to be done before the investigation can be said to 

 have been exhausted. 



In June, 1812, Mr. James Sowerby, in the first volume of the ' Mineral Conchology 

 of Great Britain,' while describing his Terebralida punctata, says : " The very minute 

 punctures are arranged in undulating lines. These, although to be found in most of the 

 smoother species under the usual coat, are most conspicuous on the surface in this." 



In 183G, in his new edition of Lamarck's ' Histoire naturelle des Animaux sans 

 Vertebras,' Deshayes says : " Les coquilles des Terebratules ont une structure particu- 

 liere. Lorsqu'on les examine a un grossissement as>ez considerable, on leur trouve un 

 test qui semble poreux, qui est finement pointillie, et dont les pointillures ont une forme 

 et une disposition particuliere dans chacune des especes ; il serait curieux et interessant 

 de donner a l'appui des determinations despeces la representation grossie d'une partie 

 du test, et nous sommes convaincu que ce moyen employe avec patience et perseverence, 

 contribuerait puissamment a etablir des distinctions precises entre les especes dont les 

 formes exterieures sont tellement rapprochees que la plupart des auteurs les confondent 

 sous un petit nombre de denominations communes." 



In 1848 Alcide d'Orbigny, after the reading of a paper of mine relating to a claim 

 made by Deshayes 1 to having been the first discoverer of the punctures in the shell of 

 Terebratula, says : "'Quant a la ponctuation du test, elle est en rapport avec la forme 

 des bras ; les especes qui ont les bras libres ont le test fibreux ; le test est ponctue dans 

 celles qui ont les bras coudes ; 1'ouverture du crochet est elle meine en rapport avec la 

 forme des bras et de la ponctuation." 



bince 1843 this important inquiry has been most actively and ably pursued by Dr. 

 W. B. Carpenter and other investigators. In 1852, being fully impressed with the im- 

 portance of Dr. Carpenter's most valuable discoveries, 2 I asked my distinguished friend to 



i 'Bull. Soc. Geol. de France,' 22nd May, 18-18. 



2 "General Results of Microscopic Inquiries into the Minute Structure of the Skeletons of Mollusca, 



