138 SYNOPSIS OF THE CHARACTERS OF THE 



is one of those very interesting forms which, from being aberrant in their characters, form the passage from one 

 group to another, being, in fact, a connecting link between tlie two. The existence of those links in nature 

 has been frequently urged by short-sighted observers as a reason for uniting the groups so connected. We 

 should always, however, bear in mind that no group can be considered natural or perfect which does not ex- 

 hibit forms, partaking in some measure of the characters of the allied groups, and thus shading, as it were, the 

 one into the other. When such links are wanting, we may rest assured that they are either extinct or as yet 

 undiscovered ; for, wherever our knowledge of species is extensive, we invariably find them forming a continu- 

 ous and unbroken chain of affinity, harmoniously blending one into the other by the most insensible gradations, 

 yet, nevertheless, presenting at certain points such remarkable types of form or structure as to deserve a generic 

 or sectional name, although each point or type may be shewn to pass insensibly into the next, so much so, that 

 those species which connect the two groups are frequently of such an intermediate character that it may 

 be difficult to say to which of them it really belongs^. 



The present species possesses the triangular cardinal area of the genus Cyrtia, but the beak of the dorsal 

 valve is sliMitly incurved, the mesial fold is sulcated, and altogether the characters of the species shew a decided 

 tendency to return again to the more ordinary form of the typical Sjjirifers, which it enters by means of those 

 small fusiform shells forming the subgenus Fusella, M'Coy, in which the cardinal area, though wide and curved, 

 has parallel edges ; this latter group passes into the true Spirifers by means oi' the Fusella fits if arm is on the one 

 hand, and Spirifera speciosa, and such hke shells, on the other. Length one inch, width one inch eight lines. 



Cyrtia senilis. Pliil. sp. 



Spirifera senilis. Phil. Geol. York. 



Sp. Ch. — Semicircular, irregular, gibbous; ventral valve convex; cardinal area triangular, curved, three 

 times as wide as high, strongly striated transversely ; surface with radiating strise, and large, obtuse, irregular, 

 concentric wrinkles. 



This very curious shell is among the rarest Brachiopoda of the carb. series ; the shell is very thin, as' in 

 Anomia, and the irregular sui-face also gives it a very peculiar aspect. Length two inches, width two inches 

 nine lines. 



Cyrtia simplex. Phil. sp. 



Spirifera simplex. Phil. Pal. Fos. 



Sp. Ch. — Dorsal valve pyramidal, acute; cardinal area very large, triangular, marked with decussating 

 strise ; ventral valve convex ; mesial ridge and hollow rounded, obtuse ; surface entirely smooth, without ribs. 



This shell is closely allied to the C. cuspidata, but is well distinguished by the want of lateral, radiating 

 ribs. Length one inch six lines ; width one inch eight lines. 



Cyrtia subconica. 3Iart. sp. 



C. Anomites subconicus. Mart. Pet. Derb Spirifer subconicus. Sow. Geol. Trans — Spirifera subconica. Phil. Pal. 



Fos. 



A few fragments, possibly of this species, have occurred ; it resembles the C. cuspidata in miniatm-e, but 

 with only five ribs on each side of the mesial fold. 



* A remarkable instance of the mischief produced by considering all naturally-connected groups as one, may be seen 

 in the family Unionidaj, or fresh water mussels ; in this family the species are exceedingly numerous, and consequently 

 the affinities very perfect; some of the best American and European naturalists, who have devoted years to the 

 study of these shells alone, considered it to be divided into upwards of twenty genera, all of them distinguished by 

 marked peculiarities, either in the animals or the shells, but, of course, connected by aberrant species ; this perfection. 



