VIVIPAROUS GASTEROrODS. 369 



surface. These particles, in the genus Helix, have the form 

 of regular rhomboidal crystals, which we know is the primi- 

 tive form of the crystals of carbonate of lime. The crystals 

 are so large and hue that they may be made exquisite ob- 

 jects for the microscope, and we are, whilst viewing them, 

 wrapped in admiration to conjecture by what subtle process 

 of excretion and chemical affinity the fluid from which they 

 are deposited must have been exhaled and left in a quietness 

 not to disturb the regularity of the deposition on the first 

 minute nucleus. The eggs of all the testaceous (terrestrial) 

 Mollusca, according to M. Turpin, have the exterior enve- 

 lope hardened in this manner, viz. by the addition of carbon- 

 ate of lime on its internal surface, but very few of them 

 offers the lime in the state of crystallisation exhibited in the 

 egg of the Helices ; but, on the contrary, it is, as in the eggs 

 of birds, and in the bone of vertebrate animals, deposited in 

 grains and confusedly. Such is the inference Turpin draws 

 from an analysis of the ova of the Bulimus hsemastomus and 

 Achatina variegata, among the largest of any Mollusks. The 

 eggs of all the naked snails (Limacidse) are soft, transparent, 

 and entirely destitute of the calcareous particles or crystals ; 

 and it is curious to remark that the eggs of a genus which 

 is intermediate between the shelled and naked tribes, the 

 Cryptella of the Canary Isles, have the same neutral charac- 

 ter, for the coat or shell is indeed covered interiorly with 

 numerous crystals, but the crystals are ill-formed, and 

 exhibit the rhomboid figure very imperfectly.* 



BufFon has said that all shelled Mollusca are oviparous. 

 This is true, in a physiological sense, when applied even to 

 the whole class ;-f but there are many exceptions to the 

 axiom if we take the word o\'iparous in its popular meaning 

 as signifying animals which extrude their young from the 

 body enclosed in an egg. The only classes which in this 

 latter view are unexceptionably oviparous are the Cepha- 

 lopods, the Pteropods, and Brachiopods. The progeny of 

 the Tunicata are not born until they have passed into the 

 larvated state; and I have instanced a few viviparous spe- 

 cies amongst the Bivalves, and the dioecious phytophagous 



* Turpin in Ann, dcs Sc. Nat. xxv. 426, and in Ann. des. Sc. Nat. vi. 

 (1836) 14. Part. Botaniquc. — The egg of Limax rufus lias a calcareous 

 shell according to Laurent. Ann. des So. Nat. (1835), iv. 249. 



t " Oiinie vivum ex ovo, pcut-on dire aujourd'hui avec plus de raison que 

 dans les siecles precedents. 11 n'y a plus un animal dont on ne connaissc 

 ou I'apparcil sexuel ou quelque moyen dc reproduction." — Vms Bkneden, 

 Ann. (la: Sc. Nat. (1849) xi. 13. 



B B 



