MOLLUSCOUS ANIMALS. 33 



I do not follow his system, and I vaay faacy tbat he at times sub- 

 divides too much, and indulges too much iu the invention of names 3 , 

 but we ovve to him the pressing on our notice, the importance of attention 

 to the foot, the operculum and the odontophore of Gasteropods ; much 

 assistance in estimating the value of shell characters, and the essential 

 principle that knowledge of the animal, the operculum and the 

 odontophore must always be united with that of the shell before we can 

 be satisfied as to ,its systematic relations in a natural arrangement. 

 This last principle is not always convenient in its application, and we 

 are sometimes driven to rely for the time on resemblances, which cannot 

 bo accounted certain proofs of real affinity, but even then it is well to 

 know Avhoro our information is deficient, and it is truly important that 

 where knowledge is accessible we should be excited to seek it, not 

 supposing our work to be done whilst essential points are neglected. 

 Few indeed have contributed as much as Dr. Gray to the progress of 

 this part of Malacology and let him be honored accordingly. 



I have already expressed the opinion that Cuvier's orders of G-astero- 

 pods were founded on the right principles, though increasing knowledge 

 has shown that he divided somewhat too minutely for natural grouping. 

 T':os8 who have corrected his plan seem to have gone to the other 

 extreme in uniting as one order the strikingly different Teetibranchiata 

 and Nudibranchiata, which they have immediately to admit as sub- 

 orders, and which every observer feels to be as well separated from each 

 other, as either of them from the other received orders. Correcting 

 this error we have, as already given, the five orders of Gasteropoda, 

 and it remains to consider more particularly their families. As excelling 

 in the organs of sense and in power, and as manifesting an approach 

 towards the higher classes of the sub-kingdom, the Nucleobranohiata, 

 though deviating most widely from the true Gasteropodous type, must 

 rank as the first order. Those^ known are divided into two families: 

 Firolidae with elongated bodies and either no shell, or a cap-like very 

 delicate shell enclosing the branchiae with the heart and liver on the 

 back of the animal ; and Atlantidae with a shell into which the animal 

 can withdraw and which has an operculum. Other forms probably 

 exist or had existed but are not known to us. The nearest approach 

 may possibly be found in Janthina among the Pectinibranchiates, which 

 almost imitates the muzzle of Firolidae and resembles them in dentition. 

 The branchiae partially protected by the shell are exposed on the back, 

 and the animals are pelagic and floating with the operculum converted 



