CONCLUDING SYNOPTICAL OBSERVATIONS. 217 



CONCLUDING SYNOPTICAL OBSERVATIONS. 



During the period occupied by the pubUcation of the earlier portions of this Mono- 

 graph it was suggested to rae upon more than one occasion by a palgeontologist, since 

 deceased, whose varied and extensive knowledge entitled his opinions upon such a 

 subject to high consideration, that I should reconstruct the TrigonicB by arranging them 

 into a family, separating its species into from five to ten genera. The consideration of 

 a similar proposal has probably occurred to other naturaUsts, and had, in fact, been 

 present to my own mind during many previous years, and had induced me to bestow 

 more than usual attention upon the various aspects assumed by the genus. The results 

 of these observations had, however, tended in a direction the opposite of that proposed 

 to me ; they had led to the perception of a general resemblance between the several groups 

 of species in features of sufficient importance to induce me to regard them as forming only 

 so many portions of one great whole, — as so many allied forms greatly varied, which, 

 whether viewed separately or in combination, constituted only a single generic idea, the 

 subordinate features of which were elaborated, some synchronously, others in a certain 

 order of geological succession, never occurring all together or in a single stratigraphical 

 position. It therefore appeared to me that to regard certain differences between such 

 groups as of generic value would be an attempt to dissociate forms which are by 

 natural affinity in close relationship, — to do violence to the chain of life disclosed by an 

 important genus through the great geological periods to which it belonged, — and not less 

 an endeavour to dissolve the association which exists between the more ancient Mesozoic 

 forms and the Tertiary and living portion of the genus which still remain to us. 



As examples of such proposed reconstruction grounded upon difierences of figures and 

 of surface-ornaments, it would apparently become necessary to divide the extensive section 

 of the Scabra into three genera, one type form of which would have its representative in 

 T. spinosa. Park., Plate XXIV ; a second in T. pennata, Sow., Plate XXIV ; the third in 

 T. aliformis^ Park., Plate XXV ; species remarkably distinguished when separated from 

 the TrigonicE generally and brought together for comparison, in their general forms, their 

 ornaments, and even to some extent in their internal hinge-characters, but which will be 

 found to form a gradual approximation when they are compared through the connecting 

 links of other examples of the Scabroe, — forms from which they can only possibly be 

 as so many species. 



The Quadrata also, although they are sufficiently distinct in the more short or 

 quadrate forms as T. quadrata, Ag., p. 105, and T. spectabUis, Sow., Plate XXVI, 

 present in other examples approaches both to the groups spinosa and ali/ormis of the 

 Scabra. 



