250 Mr. J. Lycett on some new species o/Trigonia 



of the Oolitic rocks. The sixth section, or Pectines, is repre- 

 sented by the living species of our Australasian seas, in which 

 both the form and ornaments of the surface differ materially from 

 those of the fossil sections ; the radiating crenulated costse and 

 toothed edges of the valves remind us of the Lima and Pectines ; 

 the very partial flattening of the posterior slope and general 

 figure nearly resembles Cardium, but the internal characters have 

 nothing peculiar. 



In tracing the vertical range of the species throughout the 

 Oolitic rocks, it will be found that the facts coincide with others 

 which have been recorded respecting the range of species per- 

 taining to the other leading genera of Conchifera ; it is rare that 

 a species ranges beyond a single formation ; and when its exist- 

 ence was further extended, it occurs usually in the newer forma- 

 tion, as a variety only, and bearing a physiognomy readily distin- 

 guished from that of the typical form. The aspect of the genus 

 seems to have undergone a very gradual but continuous change 

 throughout the secondary formations, by which the sectional 

 forms of the lower Oolites were modified in the upper Oolites, 

 and finally disappeared as the Quadrates and Scabrce of the Cre- 

 taceous rocks acquired prominence ; finally, between the latter 

 aud the recent Pectines, there occurs a chasm not less zoological 

 than stratigraphical, in which we lose the links by which pro- 

 bably they were connected. 



In the discrimination of species, it is of importance to have 

 correct ideas of the surface-markings which distinguish their 

 young condition. In the Clavellata generally, the young shells 

 have their concentric costse continued across the area ; the costse 

 are slightly tumid and projecting when they cross the position 

 of the marginal carina : in several instances the young of this 

 section have smooth undivided costse, and such species as in the 

 adult state have their costse forming an angle or undulation, do 

 not exhibit any trace of such a feature until five or more costse 

 have been formed. But if in the rudimentary condition we are 

 often unable to distinguish forms which subsequently become 

 widely separated, the individuals of a species are in their imma- 

 ture stage all alike ; they exhibit no traces of that law which is 

 afterwards developed to form varieties of a species. A common 

 feature observable in the Clavellatce and Quadrates which tends 

 to mask the species, consists in a confused or reticulated dispo- 

 sition of the tubercles, which no longer form regular concentric 

 or curved rows ; the tubercles also become irregular in size, or 

 they are partially flattened and confluent in the rows ; it is not 

 clear to what causes are to be attributed this unequal secretion 

 of shell by the mantle at the lower border ; it is however quite 

 distinct from another and final change observable in aged shells, 



