Costapex baldwinae, new species, is described from deep reef habitats of the southern and eastern Caribbean Sea, including Curaçao, Dominica and Guadeloupe, where it occurs at bathyal depths on sunken wood. It is assigned to the genus Costapex based on phylogenetic analyses using partial sequences of COI, 12S and 16S mitochondrial genes that reveal it to be the sister species of two Indo-Pacific members of this genus. This new species most closely resembles Costapex martinorum (Cernohorsky, 1986) from the Philippines, but differs in being smaller, and in having a slightly lower spire and more prominently beaded spiral sculpture. Of the Caribbean species of costellariids, it is somewhat similar to Nodicostellaria laterculata (Sowerby II, 1874), which occurs in shallower water and has a white or tan rather than dark brown shell, and also a taller spire, more prominent axial sculpture, and a more rounded aperture. It differs from Vexillum styria (Dall, 1889), with which it co-occurs, in having a broader shell with a lower spire, prosocline rather than opisthocline axial ribs, and more prominent, strongly beaded spiral cords. Costapex baldwinae differs from both these taxa in having rachidian teeth with three cusps rather than five cusps (N. laterculata) or seven cusps (V. styria). The genus Costapex was previously known only from Indo-Pacific species. The discovery of this new species represents a significant expansion of the range of this genus into the Caribbean Sea.

加勒比海南部和东部的深礁栖息地（包括库拉索岛，多米尼加和瓜德罗普岛）描述了新物种Costapex baldwinae，它们栖息在沉没深度的沉木上。根据系统发育分析，使用COI，12S和16S线粒体基因的部分序列，将其分配给Costapex属，这表明它是该属的两个印度洋-太平洋成员的姊妹物种。这种新物种与菲律宾的Costapex martinorum（Cernohorsky，1986）最相似，但区别在于较小，尖顶稍低，串珠螺旋雕塑更为突出。在加勒比种类的肋节藻中，它与Nodicostellaria Laterculata有点相似（Sowerby II，1874），它发生在较浅的水中，具有白色或棕褐色而不是深棕色的外壳，并且还有较高的尖顶，更突出的轴向雕塑和更圆的孔径。它不同于共发的Vexillum styria（Dall，1889年），具有更宽的外壳和较低的尖顶，脯氨酸而不是阿斯匹林的轴向肋骨，以及更突出的强串珠螺旋线。Costapex baldwinae与这两个类群的不同之处在于具有三尖齿而不是五个尖齿（N. Laterculata）或七个尖齿（V. styria）的棘齿。Costapex属以前仅从印度太平洋物种中才知道。这个新物种的发现代表了该属的范围在加勒比海的显着扩展。