SUMMARY PLATE XIV. 



The three preceding plates do not illustrate the biological relations of the Aietidse as a whole 

 with sufficient clearness, and this plate has been added for the purpose of supplying the defi- 

 ciency. The series of Psiloceras has been placed in what may be deemed its true position, be- 

 tween the Plicatus stock and the Levis stock ; otherwise, the arrangement is the same. The 

 resemblances of the morphological equivalents in each series can be readily seen by following 

 the forms along horizontal lines from left to right. The independence of the origin of these 

 representative forms can be studied by following up the series in vertical lines, which represent 

 descent. To a large extent, also, the more obvious differential characters which distinguish each 

 series become appreciable by the same process. 



Psil. planorbe, var. leve, Pig. 1 ; var. plicata, Fig. 2. 



Schlot. catenata, Fig. 3, is the radical of this series. 



Schlot. angulata, Fig. 24, is evidently a transition to the next species. The artist has 

 exchanged Fig. 4 with Fig. 24. 



Schlot. Charma.ssei, Fig. 5. The whorl is more involute, but the degenerate characters 

 of compression in the whorls and shallowing of the abdominal channel begin to appear. 



Schlot. Boucaultiana, Fig. 6. The involution has attained its maximum, and the degen- 

 eration of the pilse and channel is well marked. 



Wseh. curviornatum (sp. Wah.), Fig. 7, is undoubtedly distinct from Schlot. angulata, and 

 is one of the radicals of this series. 



Wseh. haploptychum (sp. Wah.), Fig. 28. The artist has exchanged Fig. 8 with Fig. 28. 



"Waeh. toxophorum (sp. Wah.), Fig. 9, is a degenerate shell, having compressed whorls, 

 and pilse crossing the abdomen, as in the rjroximate radical Wceh. curviomatum. It is, however, 

 more involute. 



Waeh. Emmerichi (sp. Wah.), Fig. 10, shows a notably involute shell, with degenerate 

 pila? and compressed whorls. 



Cal. tortile, Fig. 11, is the radical of this series. 



Cal. carusense, Fig. 12, has similar young to that of tortile below. 



Cal. Nodotianum, Fig. 13, is very similar to carusense, but with more compressed whorls 

 and better developed pilae. 



Cal. cycloides (sp. Wah.), Fig. 14, shows compressed degenerate whorls. 



Cal. Castagnolai (sp. Wall.), Fig. 15, is more degenerate than the last, but slightly 

 more involute. 



Cal. abnormilobatum (sp. Wah.), Fig. 16, is a dwarfish and more degenerate form than 

 Castagnolai, but has more iuvolute whorls. 



Cal. laqueum, Fig. 17, is an extreme form of this species, which approximates very closely to 

 a true spiratissimum. This figure is therefore placed to the right, and under Verm, spiratissimum. 



Verm, spiratissimum, Fig. 18, shows typical form, with but slight channels. 



Verm. Conybeari, Fig. 19, shows normal untuberculated variety, with stout whorls and 

 deep channels. 



Verm, ophioides, Fig. 20, exhibits the tuberculated pila? of this species. 



Psil. aphanoptychum (sp. Wah.), Fig. 21, is one of the Plicatus stock of Psiloceras. 



Psil. Kammerkarense (sp. Wah.), Fig. 22, shows the more involute and plicated form of 

 this subseries. 



Psil. mesogenos (sp. Wah.), Fig. 23, is an involute shell belonging to the true Levis stock. 1 



Arn. semicostatum, Fig. 4. The figure represents the nearly full-grown shell ; but if 

 the keel were absent, the smooth whorls of the young would closely resemble the adult whorls 

 of Psil. planorbe, var. leve. The artist has exchanged Fig. 4 with Fig. 24. 



Arn. Hartmanni, Fig. 25, exhibits young and adult characters like those of the preceding. 



Arn. tardecrescens, Fig. 26, belongs to another subseries of forms than that in which it 

 is placed, but it serves to show that quadragonal whorled shells with channelled abdomens 

 existed in this genus. 



1 Two subseries ought to have been shown here, but in trying to reduce the size of the plate the forms have 

 been placed in the same line. A similar liberty has been taken with the subseries of Caloceras and Arnioceras, but 

 this does not interfere with the truthful presentation of the general zoological relations of the forms. 



