CHAPTER VII. 



IDENTIFICATION, 



SOME shells are recognisable at a glance. No one is likely to 

 make a mistake with regard to Donax truncuhis and its square 

 end, Donax polltus and its white ray, Isocardia and its curly beaks, 

 the round Pectunculus, the fan-like Pinna, the ear-shaped Haliotis, or 

 the only British cowry. With some*pf the genera, too, the forms 

 are unmistakable. The armadillo-like Chitons, the long Solens, are 

 as distinctive as can be wished, and it would require peculiar 

 perversity to go wrong with regard to an lanthina or an Aporrhais. 

 But with the majority of shells the distinguishing even of the genus 

 is by no means so easy, and it can only be arrived at by some such 

 system of elimination, conscious or unconscious, as we have adopted. 



To show how the foregoing keys are worked let us take a shell 

 a bivalve to begin with. It is an every-day sort of specimen picked 

 up on the beach, and its valves fell apart the instant they were 

 touched,, as in dead shells they so often do. 



There is nothing peculiar about its back; there are no plates 

 across where the hinge should be and no white fingers projecting 

 inside from under the beaks ; in no way could it be said to resemble 

 a Pholas. It is just an ordinary, straightaway shell. 



Its pallial line, which is about as visible as that made by a slug, 

 is indented sharply, so that we can at once betake ourselves to 

 Section C. The valves are equal in every respect; that gives us a 

 further lift. The shell does not gape at all ; that takes us a long way 

 on. All the ligament is outside, there are three teeth in both valves, 

 the sinus is angular, and, therefore, the genus is Venus. But Venus 

 what ? Turn to the chapter in which the genera are divided into 

 species. Of Venus there are eight species. Three of these which 

 belonged to the old genus Cytherea have the inside edge of the 

 margin plain. An examination with the magnifying glass shows our 

 specimen to be finely toothed, the pattern being that which can be 

 made on pastry with a fork. The pattern does not extend along the 

 posterior side, consequently the species is not ovata. It has not the 

 fine striations and fan-like rays of gallina, nor the thick ribs and 

 white mouth of verrucosa, nor the broad thin plates and mealy look 

 of casina ; but it has flat concentric ribs and is pinkish with darker 

 pink rays, and is, in fact, the only species leftfasciata and could 

 have been at once identified by its size had we felt inclined to take 

 it that way. 



74 



