**Core Concept**
The hippocampal formation is a critical component of the limbic system, involved in the formation and consolidation of new memories. It is composed of several key structures, including the dentate gyrus, subiculum, and entorhinal cortex.

**Why the Correct Answer is Right**
The amygdaloid nucleus, while a part of the limbic system, is not considered a component of the hippocampal formation. This is because the amygdala is primarily involved in processing emotions and fear responses, rather than in the formation and consolidation of new memories. In contrast, the hippocampal formation is crucial for spatial navigation, learning, and memory. The dentate gyrus, subiculum, and entorhinal cortex work together to form a network that enables the transfer of information from the entorhinal cortex to the hippocampus proper, and ultimately to the subiculum, which then projects to various other brain regions.

**Why Each Wrong Option is Incorrect**
**Option A:** The dentate gyrus is indeed a key component of the hippocampal formation, receiving input from the entorhinal cortex and projecting to the hippocampus proper.
**Option B:** The subiculum is also a part of the hippocampal formation, serving as a transitional zone between the hippocampus proper and the entorhinal cortex.
**Option D:** The entorhinal cortex is the main input structure to the hippocampal formation, projecting to the dentate gyrus and the hippocampus proper.

**Clinical Pearl / High-Yield Fact**
The hippocampal formation is particularly vulnerable to damage from various causes, such as Alzheimer's disease, traumatic brain injury, and temporal lobe epilepsy, which can result in significant impairments in memory and spatial navigation abilities.

**✓ Correct Answer: C. Amygdaloid nucleus**