141 



succeeded in detecting them in their passage from the one to the other 

 body, although I have found, at the junctions of the adductor muscle 

 with the shells, both in Pinna and Ostrea, a layer of most elaborate 

 and complex minute vascular tissue, and amid this other vessels, few 

 in number and much greater in size. These vessels were evidently 

 not those which appertained only to the muscle itself, as they were 

 found in no other part of it, but concentrated in a single complex 

 stratum at this point. 



The periostracum, in both univalves and bivalves, is as highly or- 

 ganized as the other parts of the shell ; and in all the species which 

 I have subjected to examination, it presents every symptom of pos- 

 sessing a high degree of vitality. If it be removed from the other 

 structures by maceration in a weak solution of hydrochloric acid, 

 immersed in water between glasses, and examined by transmitted 

 light, with powers varying from 200 to 500 linear, it will be found, 

 in Mytilus, Oliva, Bulla, Cyclostoma, Cypr<ea, and numerous other 

 genera, to abound in minute ramifying vascular tissues, varying in 

 abundance, and somewhat in character, in different genera. Some- 

 times they are seen (especially in the last-formed parts of the peri- 

 ostracum) in a progressive state of development, the larger trunks 

 being apparently perfectly tubular in their structure, while the branch- 

 ing and terminating portions are more or less composed of irregular 

 moniliform strings of gelatinous molecules, as if the vessels were in 

 progress of development, in a similar manner to that which I have 

 described as occurring in the formation and development of the mi- 

 nute vascular tissues of the Corallidae, in the * Philosophical Transac- 

 tions,' part 2, 1842, page 220, pi. 17. 



The periostracum possesses also in an eminent degree, both repro- 

 ductive and reparative powers. In almost every instance that has fall- 

 en under my observation, it is found to abound in cytoblasts, varying 

 in their characters in different genera and species. In the greater 

 number of instances, they do not affect any particular mode of ar- 

 rangement, but are developed in groups over the whole surface of 

 the membranous structure wherever it needs strengthening. In some 

 cases they are seen advancing as it were in broad bands over the sur- 

 face of the organ, from the hinge towards the lip, in the manner 

 exhibited in fig. 7, PI. xvii. which represents a portion of the perios- 

 tracum of Solen vagina, which was separated from the shell by ma- 

 ceration in dilute hydrochloric acid. In the front of the line of 

 progression {a) they are observed in their earlier stages of develop- 

 ment of cellular structure, but as we recede towards the line {b) the 



