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30 JOURNAL OF CONCHOLOGY, VOL. 12;- NO. I, JANUARY, I907. 



represented each by a unique specimen. These sports really possess 

 very little scientific interest in the pi»esent state of our knowledge. 

 They are mere curiosities. 



In some groups of animals and plants there exists yet another kind 

 of abnormal forms — dimorp^is. Individuals of the same species are 

 found occurring in two forms, often very unlike each other. These 

 are quite constant and no intermediate gradations ocC'ur ; they pair 

 together and produce both fojms as their progeny. In the mollusca 

 the banded and bandless forms of many land shells may be con- 

 sidered as a not very stroiig^ marked form of dimorphism or poly- 

 morphism. I know of no more characteristic example, but the 

 possibility of the occurrence of such must be borne in mind. 



The various deviations from the normal that are produced by 

 direct injury or by disease are worthy of mention. These we may 

 perhaps call pathological. In the case of some long, slender shells 

 the apex has so frequently and for so many generations been 

 subjected to injury, that at length the species has acquired the 

 habit of making preparation for it. In Turritellce. for example the 

 apical one-fourth or so of the shell will be found to be unoccupied 

 by the animal which has shut off the upper whorls by a series of 

 successive septa occurring about every half-turn. In Trimcatella 

 and other forms the process is carried still further, for before the 

 septun is made the shell about to be vacated is so thinned that it is 

 soon broken off and the adult is always decollated. 



Having now briefly considered the various groups under which 

 may be classed the different modifications that are to be found 

 within the limits of a species, there remain only a few practical 

 points to be discussed. 



A very important matter is the establishment of new species. A 

 shell brought from some little known region^ if only it shows some 

 slight difference from forms already described is generally far too 

 hastily assumed to be specifically distinct. I think that anyone who 

 has carefully followed my remarks upon aberrant forms will see that 

 it must always be a somewhat doubtfully justifiable proceeding to 

 establish a species upon a single specimen. Even if a number of 

 specimens showing constant characters are obtained, whenever these 

 show a pretty close relationship to a known species from a neighbour- 

 ing region, the probability is strong that it will prove ultimately to be 

 only varietally separable. Each year our already heavily burdened 

 lists are being increased by tlie addition of hundreds of so-called new 

 species, many, perhaps most of which are stated to resemble other 

 already known species. Although I have before stated that each 

 writer must be allowed to have and to hold his own standard of the 

 meaning and significance of the word species it by no means follows 



