APERTURE. 69 



posterior side. The anterior of a spiral univalve is that part of 

 the outer lip which is at the greatest distance from the apex. Of 

 a symmetrical conical univalve such as Patella, it is that part 

 where the head of the animal lies, indicated by the interruption 

 of the muscular impression. Of cirripedes, that part where the 

 cilise protrude are anterior ; of Brachiopoda, that part which 

 is farthest from the umbones and which corresponds with the 

 ventral margin in other Bivalves. The anterior of symmetrical, 

 convolute univalves, is the outer or dorsal part of the aperture, or 

 that part which is farthest from the spire. Lamarck and other 

 Conchological writers have occasioned much confusion by their 

 errors on this subject ; describing the same part of a shell at 

 one time anterior, at another posterior ; but generally the reverse 

 of the above arrangement, which is founded upon the natural 

 position of the animal, and generally adopted. The anterior will 

 be indicated by the letter a, in figs. 119, 421, 229, 34, 202. 



ANTIGONA. Schum. A genus composed of Venus cancellata, 

 Lam. (fig- 119.) and similar species. 



ANTIQUATED. This word, signifying out of date, is occasionally 

 used to express that species of composition which constantly 

 occurs in shells, by each fresh deposit or layer of calcareous 

 matter, forming a new margin, which being replaced by its suc- 

 cessor, is no longer used as the margin, and is consequently said 

 to be out of date. 



APEX. This term does not apply to the natural position of a 

 shell, but is used in a mathematical sense, to indicate the nucleus 

 or first formed part ; which may be considered as the point of 

 the spiral cone. From this point, the shell enlarging rapidly or 

 slowly as it descends, takes a spiral, arched, straight, oblique, 

 convolute, or irregularly spiral course. The apex will be in- 

 dicated by the letter a, in fig. 282 and 466. 



APERTURE or MOUTH. The entrance to the spiral cavity of 

 univalve shells. The parts of the aperture are separately de- 

 scribed, as follows : The inner lip or labium is that part which 

 lies over the preceding whorl of the shell. It terminates an- 

 teriorly, or towards the lower part in what is termed the columella, 



