290 Messrs. W. K. Parker and T. Rupert Jones on some 



frequently uniting at the umbilicus to form a central disc or 

 group of granules. External edge of the chambers frequently 

 bordered by a keel of exceedingly variable character. Surface 

 of the chambers usually smooth, occasionally granulated. 



The specimens here figured are smooth and subcarinate, but 

 the condition of their keel is very inconstant ; they are prominent 

 at the umbilici, but have no thickened umbilical discs; nor 

 are the septa thickened or raised, though (as usual) visible 

 externally. 



Aperture at the upper angle of the chamber, somewhat pro- 

 duced, and variable ; sometimes presenting a round hole, sur- 

 rounded by numerous slight fissures (as in fig. 1 ) ; sometimes 

 the round aperture is slit on the lower side, and becomes ovato- 

 triangular, or even roughly spade-shaped, as in fig. 12. 



This variability of the form of the aperture, which is also 

 observable in many of the individuals figured by authors, and 

 among the Cristellarice of the London Clay and other deposits, 

 destroys the presumed generic, or even specific, value of " Ro- 

 bulina.'' 



The very beautiful series of figures in Fichtel and MolFs 

 Testacea microscopica aliaque minuta, &c. (1803) present well- 

 marked individuals of the chief varieties of this delicate and 

 symmetrical shell. Numerous conditions of surface are here 

 apparent : polished and granulate ; smooth and costate ; smooth- 

 ribbed and granular-ribbed ; keeled, partially keeled, and keel- 

 less ; serrate-keeled, tooth-keeled, and merely spined without a 

 keel, — these are the variable and interchanged characters, the 

 several groupings of which in individuals these authors recog- 

 nized in their varieties a to fju; and by which D'Orbigny en- 

 deavoured to arrange these varieties into specific groups. Well 

 might the former despair of finding a specific character distinc- 

 tive of these varieties*; and, although some of D^Orbigny's 

 binomial terms are not without an incidental value, yet, as far 

 as the specific relations of this shell are concerned, we prefer 

 to follow Fichtel and Moll in reverting to Linnseus^s decision, 

 who, seizing the most characteristic variety f, invested it with the 



♦ " Forma hujus speciei ita com])arata est ejusque varietates tam multi- 

 plices, ut fere impossibile videatur characterera specificum erui posse, quo 

 haec species a reliquis congeneribus distingueretur et tamen nulla varietas 



excluderetur Ad evitandam omuem, quae exinde exoriri 



posset, ambiguitatem et confusionem, optimum nobis visum est, ex multi- 

 lariis varietatibus pracipuas eligi, earumque unam, secundum quam Lin- 

 naeus sine dubio nomen suum composuerit, quoad omnes proprietates 

 primura describi, atque rehquas solummodo quatenus afFerri, quatenus 

 quaelibet a ceteris discedat." — Op. cit. pp. 69 & 70. 



t The acceptance of the tooth-keeled and rowel-like (or the most de- 

 veloped) form as the most characteristic variety, appears to us to be quite 

 correct. 



