65 



research. The fact of the mammillated external appearance of 

 this species is easily seen and as easily recorded : so is also the 

 fact of the striated internal appearance. But it would be difficult, 

 by any description, to make it clearly understood how such ap- 

 parently contradictory appearances could result from the folding 

 of a simple membrane*, until it had been ascertained, by careful 

 and multiplied dissections, that this and all other characters of 

 the fold of the membrane forming the wall of the pouch are 

 grounded upon one single and simple unity of plan of which all 

 present clear and intelligible modifications. 



On the outside of this species as usually seen in a fossil state, 

 and as it must usually have appeared in a recent state, no mark 

 of the plait is seen. Large rounded elevations scattered, at first 

 sight irregularly, over the whole surface, are all that meet the 

 eye. If however the reader will carefully consider figs. 13 and 

 14 of PI. XIII. he will see, in fig. 13, the simple plaits, regular and 

 uninterrupted, within and without, as in V. striatus-, while in 

 fig. 14 he will see that the perfect plait is still present, and that 

 the inner surface [the lower part of the figure] is still simple and 

 uninterrupted, but that the outer plaits are interrupted along 

 their whole length by rounded elevations, not solid, but hollow, 

 elevations of the membrane into those shapes instead of the 

 plait being plain and simple. These figures are somewhat ex- 

 aggerated in size in order that the principle may be more clearly 

 understood. 



It will be found that in each of the forms which follow, and 



* As the extraordinary complications in the fold of the membrane of 

 many species were gradually developed by multiplied dissections, 1 long de- 

 spaired of being able, by any descriptions, to make them understood. When 

 by degrees the unity of plan which I have endeavoured to indicate above, 

 as the groundwork of all that complexity, opened upon me, t felt an im- 

 portant key to have been obtained, which experiment proved to be appli- 

 cable to every case. I indulge some hope that the development of this 

 unity may be of a utility beyond the mere understanding of the forms now 

 under discussion however interesting those may be to myself and Others. 

 In the descriptions of numerous tissues in human and other branches of 

 anatomy, I have often myself felt the want of some clear and simple basis 

 of unity in the descriptions attempted of those tissues ; which want has 

 caused them to be often unintelligible. Any pains which the investigation 

 of the present subject may have cost will be more than rewarded if the sug- 

 gestion of an unfailing unity in the arrangement of all complex tissues shall 

 be felt to be (as I cannot doubt that it is) generally applicable. It seems to 

 me, that as, by application of this principle, forms varying so entirely as it 

 will be found that many of the Ventriculidae do in mere general external cha- 

 racters, and therefore heretofore classed in entirely different natural groups, 

 are now demonstrated to belong to one group, so the application of the like 

 principle may, in many points of anatomy, lead to the discovery of intimate 

 relations, not now suspected, between tissues which appear very different 

 in structure. 



